Updates widget tests for the new MultiFeedProvider architecture:

- FakeMultiFeedProvider replaces FakeFeedProvider
- Supports per-feed state management (FeedType parameter)
- Uses sentinel-compatible copyWith for state mutations
- Tests cover both authenticated and unauthenticated flows
- Tests for PageView swipe navigation when authenticated
- Tests for single-feed display when not authenticated

Removes orphaned test/providers/feed_provider_test.dart that
referenced the deleted FeedProvider.

Updates test/widget_test.dart counter test with provider setup.

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

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

Refactors FeedScreen to work with MultiFeedProvider and adds several
UX and reliability improvements:

PageView for feed switching:
- Authenticated users can swipe between Discover and For You
- Unauthenticated users see only Discover (no PageView)
- Per-feed ScrollControllers with position restoration

Auth state synchronization:
- Listens to AuthProvider changes
- Jumps PageController to page 0 on sign-out to match provider state
- Prevents tab/page mismatch after re-authentication

Lazy feed loading (_ensureFeedLoaded):
- Triggers initial load when switching to an unloaded feed
- Handles case where user signs in after app start and taps For You
- Called from both tab tap and swipe navigation

This fixes issues where:
- For You tab showed empty state after signing in mid-session
- PageController stayed on page 1 after sign-out while provider
switched to Discover, causing misalignment on re-auth

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

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

Extracts feed rendering logic into a reusable FeedPage widget that
handles all feed states:

- Loading: Centered CircularProgressIndicator
- Error: User-friendly message with Retry button
- Empty: Contextual message based on auth state
- Posts: ListView.builder with pagination support

Key improvements:
- Empty state now wrapped in RefreshIndicator with CustomScrollView
and SliverFillRemaining, allowing pull-to-refresh when feed is empty
- Error messages are user-friendly (maps technical errors to readable text)
- Loading more indicator at list bottom during pagination
- Proper scroll controller integration for infinite scroll

This fixes an issue where unauthenticated users with an empty Discover
feed had no way to retry without restarting the app.

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

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

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>