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>

tests/test_allowed_types.py

@@ -83,6 +83,8 @@ class TestGotenbergCoverageDocuments:
         ".tif",
         ".webp",
         ".svg",
+        ".heic",
+        ".heif",
     }
     _html_extensions = {".html", ".htm"}
     _markdown_extensions = {".md", ".markdown"}