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/components/fileEditor/FileEditorThumbnail.tsx

@@ -9,6 +9,7 @@ import VisibilityIcon from '@mui/icons-material/Visibility';
 import UnarchiveIcon from '@mui/icons-material/Unarchive';
 import PushPinIcon from '@mui/icons-material/PushPin';
 import PushPinOutlinedIcon from '@mui/icons-material/PushPinOutlined';
+import LockOpenIcon from '@mui/icons-material/LockOpen';
 import DragIndicatorIcon from '@mui/icons-material/DragIndicator';
 import { draggable, dropTargetForElements } from '@atlaskit/pragmatic-drag-and-drop/element/adapter';
 import { StirlingFileStub } from '@app/types/fileContext';
@@ -56,7 +57,14 @@ const FileEditorThumbnail = ({
   isSupported = true,
 }: FileEditorThumbnailProps) => {
   const { t } = useTranslation();
-  const { pinFile, unpinFile, isFilePinned, activeFiles, actions: fileActions } = useFileContext();
+  const {
+    pinFile,
+    unpinFile,
+    isFilePinned,
+    activeFiles,
+    actions: fileActions,
+    openEncryptedUnlockPrompt,
+  } = useFileContext();
   const { state } = useFileState();
   const hasError = state.ui.errorFileIds.includes(file.id);
 
@@ -77,6 +85,7 @@ const FileEditorThumbnail = ({
   const isZipFile = zipFileService.isZipFileStub(file);
 
   const pageCount = file.processedFile?.totalPages || 0;
+  const isEncrypted = Boolean(file.processedFile?.isEncrypted);
 
   const handleRef = useRef<HTMLSpanElement | null>(null);
 
@@ -301,6 +310,21 @@ const FileEditorThumbnail = ({
 
         {/* Action buttons group */}
         <div className={styles.headerActions}>
+          {isEncrypted && (
+            <Tooltip label={t('encryptedPdfUnlock.unlockPrompt', 'Unlock PDF to continue')}>
+              <ActionIcon
+                aria-label={t('encryptedPdfUnlock.unlockPrompt', 'Unlock PDF to continue')}
+                variant="subtle"
+                className={styles.headerIconButton}
+                onClick={(e) => {
+                  e.stopPropagation();
+                  openEncryptedUnlockPrompt(file.id);
+                }}
+              >
+                <LockOpenIcon fontSize="small" />
+              </ActionIcon>
+            </Tooltip>
+          )}
           {/* Pin/Unpin icon */}
           <Tooltip label={isPinned ? t('unpin', 'Unpin File (replace after tool run)') : t('pin', 'Pin File (keep active after tool run)')}>
             <ActionIcon

frontend/src/core/components/shared/EncryptedPdfUnlockModal.tsx

@@ -0,0 +1,85 @@
+import { Modal, Stack, Text, Button, PasswordInput, Group } from '@mantine/core';
+import { useTranslation } from 'react-i18next';
+import { type KeyboardEventHandler } from 'react';
+import { Z_INDEX_OVER_FULLSCREEN_SURFACE } from '@app/styles/zIndex';
+
+interface EncryptedPdfUnlockModalProps {
+  opened: boolean;
+  fileName?: string;
+  password: string;
+  errorMessage?: string | null;
+  isProcessing: boolean;
+  onPasswordChange: (value: string) => void;
+  onUnlock: () => void;
+  onSkip: () => void;
+}
+
+const EncryptedPdfUnlockModal = ({
+  opened,
+  fileName,
+  password,
+  errorMessage,
+  isProcessing,
+  onPasswordChange,
+  onUnlock,
+  onSkip,
+}: EncryptedPdfUnlockModalProps) => {
+  const { t } = useTranslation();
+
+  const handleKeyDown: KeyboardEventHandler<HTMLInputElement> = (event) => {
+    if (event.key === 'Enter' && !isProcessing && password.trim().length > 0) {
+      onUnlock();
+    }
+  };
+
+  return (
+    <Modal
+      opened={opened}
+      onClose={onSkip}
+      title={t('encryptedPdfUnlock.title', 'Remove password to continue')}
+      centered
+      size="md"
+      closeOnClickOutside={!isProcessing}
+      closeOnEscape={!isProcessing}
+      zIndex={Z_INDEX_OVER_FULLSCREEN_SURFACE}
+    >
+      <Stack gap="md">
+        <Text fw={600} ta="center">{fileName}</Text>
+        <Text c="dimmed" ta="center">
+          {t(
+            'encryptedPdfUnlock.description',
+            'This PDF is password protected. Enter the password so you can continue working with it.'
+          )}
+        </Text>
+
+        <Stack gap={4}>
+          <PasswordInput
+            label={t('encryptedPdfUnlock.password.label', 'PDF password')}
+            placeholder={t('encryptedPdfUnlock.password.placeholder', 'Enter the PDF password')}
+            value={password}
+            onChange={(event) => onPasswordChange(event.currentTarget.value)}
+            onKeyDown={handleKeyDown}
+            disabled={isProcessing}
+            autoFocus
+          />
+          {errorMessage ? (
+            <Text c="red" size="sm">
+              {errorMessage}
+            </Text>
+          ) : null}
+        </Stack>
+
+        <Group justify="space-between">
+          <Button variant="light" color="var(--mantine-color-gray-8)" onClick={onSkip} disabled={isProcessing}>
+            {t('encryptedPdfUnlock.skip', 'Skip for now')}
+          </Button>
+          <Button onClick={onUnlock} loading={isProcessing} disabled={password.trim().length === 0}>
+            {t('encryptedPdfUnlock.unlock', 'Unlock & Continue')}
+          </Button>
+        </Group>
+      </Stack>
+    </Modal>
+  );
+};
+
+export default EncryptedPdfUnlockModal;