Enable loading user profiles via ?user= handle in the URL.

Changes:
- Add get_query_param() FFI function in effects.gleam
- Modify init() to check for user param and auto-fetch profile
- Add comprehensive tests for query param extraction and init behavior
- Clean up unused imports and dead test code
- Reorganize tests by module (effects_test.gleam)

All 92 tests passing.

- Extract profile state + view to gpreview/profile/ (profile_view.gleam, profile_actions.gleam)
- Extract feed state + view to gpreview/feed/ (feed_view.gleam, feed_actions.gleam)
- Simplify views.gleam to delegate to profile_view and feed_view
- All types re-exported from gpreview/types for compatibility
- Tests pass (102 tests)