Add automatic unlock prompt for encrypted PDFs (#4912)

## Summary
- propagate an `isEncrypted` flag from thumbnail generation into
processed file metadata so uploads know when a password is still present
- add queueing logic inside `FileContext` that detects encrypted
uploads, prompts the user via a new modal, and automatically runs the
Remove Password endpoint to replace the file and preserve history
- introduce a dedicated `EncryptedPdfUnlockModal` component that mirrors
existing styling and messaging for unlocking PDFs

## Testing
- npm run typecheck:core


------
[Codex
Task](https://chatgpt.com/codex/tasks/task_b_6919a0a418bc8328b886ec76a28170b7)

frontend/src/core/utils/thumbnailUtils.ts

@@ -5,6 +5,7 @@ export interface ThumbnailWithMetadata {
   pageCount: number;
   pageRotations?: number[]; // Rotation for each page (0, 90, 180, 270)
   pageDimensions?: Array<{ width: number; height: number }>;
+  isEncrypted?: boolean;
 }
 
 interface ColorScheme {
@@ -451,7 +452,7 @@ export async function generateThumbnailWithMetadata(file: File, applyRotation: b
     if (error instanceof Error && error.name === "PasswordException") {
       // Handle encrypted PDFs
       const thumbnail = generateEncryptedPDFThumbnail(file);
-      return { thumbnail, pageCount: 1 };
+      return { thumbnail, pageCount: 1, isEncrypted: true };
     }
 
     const thumbnail = generatePlaceholderThumbnail(file);

frontend/src/core/utils/toolErrorHandler.ts

@@ -2,6 +2,8 @@
  * Standardized error handling utilities for tool operations
  */
 
+import { normalizeAxiosErrorData } from '@app/services/errorUtils';
+
 /**
  * Default error extractor that follows the standard pattern
  */
@@ -30,4 +32,36 @@ export const createStandardErrorHandler = (fallbackMessage: string) => {
     }
     return fallbackMessage;
   };
+};
+
+/**
+ * Handles password-related errors with status code checking
+ * @param error - The error object from axios
+ * @param incorrectPasswordMessage - Message to show for incorrect password (typically 500 status)
+ * @param fallbackMessage - Message to show for other errors
+ * @returns Error message string
+ */
+export const handlePasswordError = async (
+  error: any,
+  incorrectPasswordMessage: string,
+  fallbackMessage: string
+): Promise<string> => {
+  const status = error?.response?.status;
+
+  // Handle specific error cases with user-friendly messages
+  if (status === 500) {
+    // 500 typically means incorrect password for encrypted PDFs
+    return incorrectPasswordMessage;
+  }
+
+  // For other errors, try to extract the message
+  const normalizedData = await normalizeAxiosErrorData(error?.response?.data);
+  const errorWithNormalizedData = {
+    ...error,
+    response: {
+      ...error?.response,
+      data: normalizedData
+    }
+  };
+  return extractErrorMessage(errorWithNormalizedData) || fallbackMessage;
 };