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/app/(tabs)/_layout.tsx

@@ -67,6 +67,15 @@ export default function TabLayout() {
           headerTitle: "Profile",
         }}
       />
+      {/* File detail screen – hidden from tab bar, accessed via navigation */}
+      <Tabs.Screen
+        name="file-detail"
+        options={{
+          href: null,
+          title: "File Details",
+          headerTitle: "File Details",
+        }}
+      />
     </Tabs>
   );
 }

mobile/app/(tabs)/file-detail.tsx

@@ -0,0 +1,4 @@
+/**
+ * File detail route – displays processing status and logs for a single file.
+ */
+export { default } from "../../src/screens/FileDetailScreen";