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>

docs/UserGuide.md

@@ -87,7 +87,7 @@ DocuElevate provides multiple convenient ways to upload documents to the system.
 
 #### Supported File Types
 - **Documents**: PDF, Word (.doc, .docx), Excel (.xls, .xlsx), PowerPoint (.ppt, .pptx)
-- **Images**: JPEG, PNG, GIF, BMP, TIFF, WebP, SVG
+- **Images**: JPEG, PNG, GIF, BMP, TIFF, WebP, SVG, HEIC, HEIF
 - **Text**: Plain text (.txt), CSV, RTF, HTML, XML, Markdown
 - **Maximum file size**: 500MB per file