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/theme/app_theme.dart

@@ -30,6 +30,37 @@ class AppTheme {
     Color(0xFF2E7D32), // Emerald Green
   ];
 
+  // Flutter's default Scrollbar renders a ~6px hairline that's easy to miss
+  // and hard to grab. This tunes it to read like Android's standard
+  // draggable fast-scroll thumb: thicker, pill-shaped, and always at least
+  // faintly visible (not just flashing in on scroll), while still getting
+  // a touch more visible/opaque while actively hovered or dragged.
+  static ScrollbarThemeData _scrollbarTheme(ColorScheme colorScheme) {
+    final thumbColor = colorScheme.onSurfaceVariant;
+    return ScrollbarThemeData(
+      thickness: WidgetStateProperty.resolveWith((states) {
+        if (states.contains(WidgetState.dragged) ||
+            states.contains(WidgetState.hovered)) {
+          return 10.0;
+        }
+        return 8.0;
+      }),
+      radius: const Radius.circular(8),
+      thumbColor: WidgetStateProperty.resolveWith((states) {
+        if (states.contains(WidgetState.dragged)) {
+          return thumbColor.withValues(alpha: 0.9);
+        }
+        if (states.contains(WidgetState.hovered)) {
+          return thumbColor.withValues(alpha: 0.8);
+        }
+        return thumbColor.withValues(alpha: 0.6);
+      }),
+      thumbVisibility: const WidgetStatePropertyAll(true),
+      trackVisibility: const WidgetStatePropertyAll(false),
+      interactive: true,
+    );
+  }
+
   static ThemeData light(
     Color seedColor, {
     ColorScheme? dynamicScheme,
@@ -49,6 +80,7 @@ class AppTheme {
       scaffoldBackgroundColor: colorScheme.surface,
       pageTransitionsTheme: _pageTransitionsTheme,
       sliderTheme: _sliderTheme,
+      scrollbarTheme: _scrollbarTheme(colorScheme),
       textTheme: GoogleFonts.interTextTheme(ThemeData.light().textTheme)
           .copyWith(
             headlineMedium: GoogleFonts.inter(
@@ -115,6 +147,7 @@ class AppTheme {
       scaffoldBackgroundColor: colorScheme.surface,
       pageTransitionsTheme: _pageTransitionsTheme,
       sliderTheme: _sliderTheme,
+      scrollbarTheme: _scrollbarTheme(colorScheme),
       textTheme: GoogleFonts.interTextTheme(ThemeData.dark().textTheme)
           .copyWith(
             headlineMedium: GoogleFonts.inter(