Links Fix – Profile Deep Link Bug

Issue

Clicking any user profile link (e.g., from a post, notification, or shared URL) always opened the current user’s profile instead of the target profile.
Root cause: GoRouter tab shell state was local to HomeShell (an IndexedStack driven by _currentIndex), so context.go('/u/<handle>') left the UI stuck on the “Profile” tab (current user view) even when the route was for another user.
Backend GetProfile endpoint only returned full profile data by user ID; when the app requested by handle it fell back to the signed‑in user, reinforcing the wrong profile display.

Frontend routing:
- Replaced manual IndexedStack shell with GoRouter StatefulShellRoute.indexedStack; tab index now follows router state.
- /profile (current user) is a shell branch; /u/:username is a separate root route that renders ViewableProfileScreen so it no longer competes with the shell tab.
- HomeShell now uses the GoRouter navigation shell (no local _currentIndex) and passes branch index to quips for playback pausing.
- ViewableProfileScreen ownership check now compares both authenticated user id and handle against the requested handle.
Backend:
- GetProfile now accepts ?handle=<username> and resolves in order: handle → :id → auth user.
- GetProfileByHandle returns full profile fields (id, handle, display_name, bio, avatar_url, origin_country, onboarding flag, created_at) so clients can render the viewed user.

Frontend: lib/routes/app_routes.dart, lib/screens/home/home_shell.dart, lib/screens/auth/auth_gate.dart, lib/screens/auth/category_select_screen.dart, lib/screens/quips/feed/quips_feed_screen.dart, lib/screens/profile/viewable_profile_screen.dart.
Backend: internal/handlers/user_handler.go, internal/repository/user_repository.go.

Backend binary rebuilt at /opt/sojorn/go-backend/bin/sojorn-api (tests pass). Needs sudo to copy over /opt/sojorn/bin/sojorn-api and restart sojorn-api.service.
Frontend changes live in the local workspace; rebuild/deploy the app to pick up the routing fix.