Grand Unification: centralize design tokens, add sponsored PostViewMode, delete 9 dead files
Phase 1 - Design Tokens:
- tokens.dart: Add NSFW, sponsored, overlay, surface state color tokens
- tokens.dart: Add granular spacing (s6, s8) and card gap constants
- tokens.dart: Add SojornRadii.xl
- app_theme.dart: Bridge new tokens (nsfwWarning*, sponsored*, overlay*, media*)
Phase 2 - Super Post Unification:
- post_view_mode.dart: Add PostViewMode.sponsored enum value
- sojorn_post_card.dart: Handle sponsored mode with badge, use token refs for NSFW colors
- feed_sojorn_screen.dart: Replace SponsoredPostCard with sojornPostCard(mode: sponsored)
- post_body/media/link_preview: Add sponsored case to all PostViewMode switches
- post_media.dart: Replace inline Colors.* with SojornColors/AppTheme tokens
- post_header.dart: Replace Colors.white with SojornColors.basicWhite
- post_menu.dart: Replace Colors.red with SojornColors.destructive
- post_actions.dart: Replace Colors.transparent with SojornColors.transparent
Phase 3 - Tree Shake (9 files deleted):
- widgets/post/sponsored_post_card.dart (absorbed into sojornPostCard)
- widgets/post/chain_quote_widget.dart (duplicate, unused)
- widgets/post_with_video_widget.dart (dead)
- widgets/video_player_widget.dart (dead)
- widgets/video_thumbnail_widget.dart (dead)
- widgets/composer_field.dart (dead)
- widgets/compose_fab.dart (dead)
- widgets/category_tile.dart (dead)
- theme/sojorn_feed_palette.dart (deprecated, unused)
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