refactor(ssrf): default enum MEDIUM prevents OFF=false (#4280)

# Description of Changes

- **What was changed**
  - **URL to PDF flow**
- Changed `ConvertWebsiteToPDF#urlToPdf` to return `ResponseEntity<?>`
and perform a redirect (`303 SEE_OTHER`) back to `/url-to-pdf` with an
`error` query param instead of throwing exceptions.
- Added alert rendering in `url-to-pdf.html` using `param.error` for
localized error display.
- Introduced new translation key `error.invalidUrlFormat` in
`messages_en_GB.properties`.
  - **Security / SSRF**
- Migrated `ApplicationProperties.System.UrlSecurity.level` from
`String` to `SsrfProtectionLevel` enum.
- Default now set to `SsrfProtectionLevel.MEDIUM` (`// MAX, MEDIUM,
OFF`).
- This avoids the issue where setting `OFF` returned `false` in
configuration parsing.
- Updated `SsrfProtectionService#parseProtectionLevel` accordingly
(using `level.name()`).
  - **Repo hygiene**
    - Added `**/LOCAL_APPDATA_FONTCONFIG_CACHE/**` to `.gitignore`.

- **Why the change was made**
- Provide user-friendly, localized error messages instead of exposing
internal exceptions on URL-to-PDF conversions.
- Ensure SSRF protection level parsing is type-safe and consistent—`OFF`
can now be set without yielding a misleading `false` state.
  - Prevent unwanted fontconfig cache files from being tracked in Git.

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] 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)
- [x] I have performed a self-review of my own code
- [x] 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)

### 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.

app/common/src/main/java/stirling/software/common/model/ApplicationProperties.java

@@ -41,6 +41,7 @@ import stirling.software.common.model.oauth2.GitHubProvider;
 import stirling.software.common.model.oauth2.GoogleProvider;
 import stirling.software.common.model.oauth2.KeycloakProvider;
 import stirling.software.common.model.oauth2.Provider;
+import stirling.software.common.service.SsrfProtectionService.SsrfProtectionLevel;
 import stirling.software.common.util.ValidationUtils;
 
 @Data
@@ -390,7 +391,7 @@ public class ApplicationProperties {
         @Data
         public static class UrlSecurity {
             private boolean enabled = true;
-            private String level = "MEDIUM"; // MAX, MEDIUM, OFF
+            private SsrfProtectionLevel level = SsrfProtectionLevel.MEDIUM; // MAX, MEDIUM, OFF
             private List<String> allowedDomains = new ArrayList<>();
             private List<String> blockedDomains = new ArrayList<>();
             private List<String> internalTlds =

app/common/src/main/java/stirling/software/common/service/SsrfProtectionService.java

@@ -61,9 +61,9 @@ public class SsrfProtectionService {
         };
     }
 
-    private SsrfProtectionLevel parseProtectionLevel(String level) {
+    private SsrfProtectionLevel parseProtectionLevel(SsrfProtectionLevel level) {
         try {
-            return SsrfProtectionLevel.valueOf(level.toUpperCase());
+            return SsrfProtectionLevel.valueOf(level.name());
         } catch (IllegalArgumentException e) {
             log.warn("Invalid SSRF protection level '{}', defaulting to MEDIUM", level);
             return SsrfProtectionLevel.MEDIUM;
@@ -215,7 +215,8 @@ public class SsrfProtectionService {
                 return false;
             }
         }
-        // For IPv4-mapped IPv6 addresses, bytes 10 and 11 must be 0xff (i.e., address is ::ffff:w.x.y.z)
+        // For IPv4-mapped IPv6 addresses, bytes 10 and 11 must be 0xff (i.e., address is
+        // ::ffff:w.x.y.z)
         return addr[10] == (byte) 0xff && addr[11] == (byte) 0xff;
     }