Replaces FeedProvider with MultiFeedProvider that manages separate
state for Discover and For You feeds. Key improvements:

Architecture:
- Per-feed state management using Map<FeedType, FeedState>
- Centralized _fetchFeed helper eliminates code duplication
- Auth-aware feed switching (For You requires authentication)
- Minute-based time updates for relative timestamps

Security (cross-session data leak prevention):
- Captures session DID before fetch to detect auth changes
- Discards For You responses if session changed during fetch
- Guards both success and error paths to prevent stale data
- Removes feed state entirely on session change (not copyWith)

This prevents scenarios where:
- User signs out during fetch → old data reappears
- User A signs out, User B signs in → User A's feed shown to B
- Fetch errors after sign-out → stale posts restored

Removes the old single-feed FeedProvider.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Introduces FeedState as an immutable state container for per-feed data
(Discover and For You feeds). Key features:

- Holds posts, cursor, loading states, error, scroll position, and
last refresh time
- Uses sentinel pattern for copyWith to distinguish "not provided"
from "explicitly set to null" for nullable fields (cursor, error,
lastRefreshTime)
- Enables proper clearing of fields on refresh/error recovery

The sentinel pattern fixes a bug where nullable fields couldn't be
cleared back to null through copyWith - critical for pagination
cursor handling when reaching the end of a feed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Move provider initialization from postFrameCallback to didChangeDependencies
for synchronous access before first build. Create ScrollController with
initialScrollOffset set to cached position, eliminating the visible flash
from loading → content at top → jump to cached position.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

View.of(context) was being called in didChangeMetrics() during widget
deactivation, causing "Looking up a deactivated widget's ancestor"
errors. The context becomes invalid before mounted becomes false.

Fixed by caching the FlutterView reference in didChangeDependencies()
for both _ReplyScreenState and _ReplyToolbarState, then using the
cached reference in didChangeMetrics().

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Add create post functionality with community picker:

Features:
- Community models for API responses
- listCommunities and createPost API endpoints
- Community picker screen with search and pagination
- Create post screen with URL validation and input limits
- Optimistic post navigation support

Tests:
- 76 tests covering models, API, and widget behavior

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Comprehensive test coverage for community features:

Unit tests:
- CommunitiesResponse, CommunityView JSON parsing
- CreatePostResponse, ExternalEmbedInput serialization
- SelfLabels, SelfLabel const construction and serialization
- CovesApiService.listCommunities with pagination/errors
- CovesApiService.createPost with validation/errors

Widget tests:
- CreatePostScreen UI elements and form validation
- Character limits enforcement (title: 300, content: 10000)
- NSFW toggle, URL field show/hide behavior

Logic tests:
- Community search filtering (name, displayName, description)
- Member count formatting (K/M suffixes)
- Description line building with separators

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Add isOptimistic flag to PostDetailScreen to skip initial comment
loading for newly created posts that haven't been indexed yet.

Also adds AlwaysScrollableScrollPhysics for consistent pull-to-refresh.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>