refactor: unify post widgets, profile screens, and tree-shake dead code
Phase 1 - Post Widget Unification:
- Added PostViewMode.thread for compact thread styling
- Updated SojornPostCard, PostBody, PostLinkPreview, PostMedia for thread mode
- Migrated feed_personal_screen, profile_screen, viewable_profile_screen to sojornPostCard
- Deleted deprecated: post_card.dart, post_item.dart, reading_post_card.dart
Phase 2 - Profile Screen Unification:
- Renamed ViewableProfileScreen to UnifiedProfileScreen
- Made handle optional (null = own profile mode)
- Added auth listener, auto-create profile, avatar actions for own profile
- Added connections navigation to stats for own profile
- Updated all routes and references across codebase
- Deleted old profile_screen.dart
Phase 3 - Tree Shaking:
- Deleted 9 orphan files: compose_and_chat_fab, sojorn_top_bar, sojorn_app_bar,
sojorn_dialog, sojorn_card, glassmorphic_quips_sheet, kinetic_thread_widget,
reaction_strip, smart_reaction_button
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