fix(api): return JSON responses for admin settings + API key endpoints to prevent Tauri client parse errors (#5437)

# Description of Changes

```console
index-DsORDqQQ.js:124 \n [TauriHttpClient] Network error: \n{url: 'http://localhost:8080/api/v1/admin/settings', method: 'PUT', errorType: 'ERR_NETWORK', originalMessage: `Failed to execute 'close' on 'ReadableStreamDefaul…cted token 'S', "Successful"... is not valid JSON`, stack: `SyntaxError: Unexpected token 'S', "Successful"...…lback (<anonymous>:284:7)\n    at <anonymous>:1:28`}\nerrorType\n: \n"ERR_NETWORK"\nmethod\n: \n"PUT"\noriginalMessage\n: \n"Failed to execute 'close' on 'ReadableStreamDefaultController': Unexpected token 'S', \"Successful\"... is not valid JSON"\nstack\n: \n"SyntaxError: Unexpected token 'S', \"Successful\"... is not valid JSON\n    at A.onmessage (http://tauri.localhost/assets/index-DsORDqQQ.js:124:22714)\n    at http://tauri.localhost/assets/index-DsORDqQQ.js:124:20748\n    at <anonymous>:272:26\n    at Object.runCallback (<anonymous>:284:7)\n    at <anonymous>:1:28"\nurl\n: \n"http://localhost:8080/api/v1/admin/settings"

index-DXbk7lbS.js:124 \n [TauriHttpClient] Network error: \n{url: 'http://localhost:8080/api/v1/user/get-api-key', method: 'POST', errorType: 'ERR_NETWORK', originalMessage: `Failed to execute 'close' on 'ReadableStreamDefaul…cted token 'a', "a72f6b26-1"... is not valid JSON`, stack: `SyntaxError: Unexpected token 'a', "a72f6b26-1"...…lback (<anonymous>:284:7)\n    at <anonymous>:1:28`}\nerrorType\n: \n"ERR_NETWORK"\nmethod\n: \n"POST"\noriginalMessage\n: \n"Failed to execute 'close' on 'ReadableStreamDefaultController': Unexpected token 'a', \"a72f6b26-1\"... is not valid JSON"\nstack\n: \n"SyntaxError: Unexpected token 'a', \"a72f6b26-1\"... is not valid JSON\n    at A.onmessage (http://tauri.localhost/assets/index-DXbk7lbS.js:124:22714)\n    at http://tauri.localhost/assets/index-DXbk7lbS.js:124:20748\n    at <anonymous>:272:26\n    at Object.runCallback (<anonymous>:284:7)\n    at <anonymous>:1:28"\nurl\n: \n"http://localhost:8080/api/v1/user/get-api-key"
```


This pull request fixes a self-hosting issue where the Tauri HTTP client
fails with `Unexpected token ... is not valid JSON` because certain API
endpoints returned plain text responses.

## What was changed

- Updated `AdminSettingsController`:
- Changed `updateSettings` and `updateSettingsSection` to return
structured JSON objects instead of raw strings.
- Standardized success and error payloads using a `Map<String, Object>`
with keys like `message` and `error`.
- Updated `UserController`:
- Changed `/api/v1/user/get-api-key` and `/api/v1/user/update-api-key`
to return JSON objects (`{ "apiKey": "..." }`) and JSON error objects
instead of plain text.

## Why the change was made

- The Tauri client expects JSON responses and attempts to parse them.
Returning plain strings like `"Successful..."` or an API key string
causes JSON parsing to fail, resulting in network errors on self-hosted
setups.

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Co-authored-by: James Brunton <jbrunton96@gmail.com>

frontend/src/proprietary/components/shared/config/configSections/apiKeys/hooks/useApiKey.ts

@@ -1,5 +1,7 @@
 import { useCallback, useEffect, useState } from "react";
 import apiClient from "@app/services/apiClient";
+import { alert } from "@app/components/toast";
+import { useTranslation } from "react-i18next";
 
 export function useApiKey() {
   const [apiKey, setApiKey] = useState<string | null>(null);
@@ -7,49 +9,107 @@ export function useApiKey() {
   const [isRefreshing, setIsRefreshing] = useState<boolean>(false);
   const [error, setError] = useState<Error | null>(null);
   const [hasAttempted, setHasAttempted] = useState<boolean>(false);
+  const { t } = useTranslation();
+
+  function failedToCreateAlert() {
+    alert({
+      alertType: "error",
+      title: t("config.apiKeys.alert.apiKeyErrorTitle", "API Key Error"),
+      body: t("config.apiKeys.alert.failedToCreateApiKey", "Failed to create API key."),
+      isPersistentPopup: false,
+    });
+  }
 
   const fetchKey = useCallback(async () => {
     setIsLoading(true);
     setError(null);
-    try {
-      // Backend is POST for get and update
-      const res = await apiClient.post("/api/v1/user/get-api-key");
-      const value = typeof res.data === "string" ? res.data : res.data?.apiKey;
-      if (typeof value === "string") setApiKey(value);
-    } catch (e: any) {
-      // If not found, try to create one by calling update endpoint
-      if (e?.response?.status === 404) {
-        try {
-          const createRes = await apiClient.post("/api/v1/user/update-api-key");
-          const created =
-            typeof createRes.data === "string"
-              ? createRes.data
-              : createRes.data?.apiKey;
-          if (typeof created === "string") setApiKey(created);
-        } catch (createErr: any) {
-          setError(createErr);
+    // Backend is POST for get and update
+    await apiClient
+      .post("/api/v1/user/get-api-key", undefined, {
+        responseType: "json",
+      })
+      .then((response) => {
+        const data = response.data;
+        const apiKeyValue = typeof data === "string" ? data : data?.apiKey;
+        if (typeof apiKeyValue === "string") {
+          setApiKey(apiKeyValue);
+        } else {
+          alert({
+            alertType: "error",
+            title: t("config.apiKeys.alert.apiKeyErrorTitle", "API Key Error"),
+            body: t("config.apiKeys.alert.failedToRetrieveApiKey", "Failed to retrieve API key from response."),
+            isPersistentPopup: false,
+          });
         }
-      } else {
-        setError(e);
-      }
-    } finally {
-      setIsLoading(false);
-      setHasAttempted(true);
-    }
+      })
+      .catch(async (e) => {
+        // If not found, try to create one by calling update endpoint
+        if (e?.response?.status === 404) {
+          await apiClient
+            .post("/api/v1/user/update-api-key")
+            .then((createRes) => {
+              const created = typeof createRes.data === "string" ? createRes.data : createRes.data?.apiKey;
+              if (typeof created === "string") {
+                setApiKey(created);
+              } else {
+                failedToCreateAlert();
+              }
+            })
+            .catch((createErr) => {
+              failedToCreateAlert();
+              setError(createErr);
+            });
+        } else {
+          alert({
+            alertType: "error",
+            title: t("config.apiKeys.alert.apiKeyErrorTitle", "API Key Error"),
+            body: t("config.apiKeys.alert.failedToFetchApiKey", "Failed to fetch API key."),
+            isPersistentPopup: false,
+          });
+          setError(e);
+        }
+      })
+      .finally(() => {
+        setIsLoading(false);
+        setHasAttempted(true);
+      });
   }, []);
 
   const refresh = useCallback(async () => {
     setIsRefreshing(true);
     setError(null);
-    try {
-      const res = await apiClient.post("/api/v1/user/update-api-key");
+    await apiClient.post("/api/v1/user/update-api-key", undefined, {
+      responseType: "json",
+      suppressErrorToast: true,
+    }).then((res) => {
       const value = typeof res.data === "string" ? res.data : res.data?.apiKey;
-      if (typeof value === "string") setApiKey(value);
-    } catch (e: any) {
+      if (typeof value === "string") {
+        alert({
+          alertType: "success",
+          title: t("config.apiKeys.alert.apiKeyRefreshed", "API Key Refreshed"),
+          body: t("config.apiKeys.alert.apiKeyRefreshedBody", "Your API key has been successfully refreshed."),
+          isPersistentPopup: false,
+        });
+        setApiKey(value);
+      } else {
+        alert({
+          alertType: "error",
+          title: t("config.apiKeys.alert.apiKeyErrorTitle", "API Key Error"),
+          body: t("config.apiKeys.alert.failedToRefreshApiKey", "Failed to refresh API key."),
+          isPersistentPopup: false,
+        });
+      }
+    }).catch((e) => {
+      alert({
+        alertType: "error",
+        title: t("config.apiKeys.alert.apiKeyErrorTitle", "API Key Error"),
+        body: t("config.apiKeys.alert.failedToRefreshApiKey", "Failed to refresh API key."),
+          isPersistentPopup: false,
+      });
       setError(e);
-    } finally {
+    }).finally(() => {
       setIsRefreshing(false);
-    }
+    });
   }, []);
 
   useEffect(() => {