feat(mobile): add pre-login legal pages, multi-image selection, file detail view, search, i18n, HEIC support

- Add Privacy Policy, Terms of Service, and Imprint links to WelcomeScreen
  and LoginScreen for GDPR/Apple compliance (pre-login access)
- Enable multiple image selection in photo library picker
- Add HEIC/HEIF image support to backend (allowed_types, convert_to_pdf, upload handler)
- Create FileDetailScreen with processing status and logs
- Add search bar to FilesScreen with debounced search
- Set up i18n with expo-localization (EN, DE, ES, FR, IT)
- Add language selector to ProfileScreen settings
- Add Imprint link to ProfileScreen legal section
- Update docs and tests

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

mobile/src/screens/UploadScreen.tsx

@@ -301,14 +301,16 @@ export default function UploadScreen() {
       mediaTypes: ["images"],
       quality: 0.9,
       allowsEditing: false,
+      allowsMultipleSelection: true,
     });
 
     if (!result.canceled && result.assets.length > 0) {
-      const asset = result.assets[0];
-      // Derive extension from MIME type so the filename matches the actual format
-      const ext = asset.mimeType?.split("/")[1]?.replace("jpeg", "jpg") ?? "jpg";
-      const filename = asset.fileName ?? `photo_${Date.now()}.${ext}`;
-      await uploadFile(asset.uri, filename, asset.mimeType ?? "image/jpeg");
+      for (const asset of result.assets) {
+        // Derive extension from MIME type so the filename matches the actual format
+        const ext = asset.mimeType?.split("/")[1]?.replace("jpeg", "jpg") ?? "jpg";
+        const filename = asset.fileName ?? `photo_${Date.now()}_${Math.random().toString(36).slice(2, 6)}.${ext}`;
+        await uploadFile(asset.uri, filename, asset.mimeType ?? "image/jpeg");
+      }
     }
   }