Add multi-account support and update project docs

- Add SavedAccount/AccountStore and rework ServerProvider around a list
  of saved accounts: switch/remove/add, swipe-to-cycle on the avatar
  button, per-account browsing prefs (sort/view/filters/cache), and a
  session-generation guard so a stale fetch from an abandoned account
  can't land in the newly active one
- Migrate existing single-account installs into the new schema
  automatically so upgrading users stay logged in
- Settings gains an Accounts section; LoginView gains an "add account"
  mode pushed on top of the main shell instead of replacing it
- Give every Scrollbar in the app a thicker, draggable, Android-style
  thumb via a shared ScrollbarTheme instead of the default hairline
- Splash screen: use the monochrome app icon (tinted per theme) and a
  smaller spinner
- Update CLAUDE.md and .claude/context/*.md to reflect the accumulated
  multi-account, sharing, and UI changes across this session, and add
  a standing instruction to keep these docs current going forward

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/views/activity_view.dart

@@ -21,7 +21,12 @@ class ActivityView extends StatelessWidget {
     final List<Widget> contentSlivers = [
       const SliverToBoxAdapter(child: SizedBox(height: 16)),
 
-      if (activities.isEmpty)
+      if (provider.isLoading && activities.isEmpty)
+        const SliverFillRemaining(
+          hasScrollBody: false,
+          child: Center(child: CircularProgressIndicator()),
+        )
+      else if (activities.isEmpty)
         SliverFillRemaining(
           hasScrollBody: false,
           child: Center(