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/main.dart

@@ -68,16 +68,30 @@ class _SplashView extends StatelessWidget {
         child: Column(
           mainAxisAlignment: MainAxisAlignment.center,
           children: [
-            ClipRRect(
-              borderRadius: BorderRadius.circular(24),
+            // The monochrome adaptive-icon layer is a plain white silhouette
+            // on transparent, meant to be tinted rather than shown as-is -
+            // srcIn recolors it to the theme's foreground so it reads
+            // correctly in both light and dark mode.
+            ColorFiltered(
+              colorFilter: ColorFilter.mode(
+                colorScheme.onSurface,
+                BlendMode.srcIn,
+              ),
               child: Image.asset(
-                'assets/icon/app_icon.png',
+                'assets/icon/app_icon_monochrome.png',
                 width: 72,
                 height: 72,
               ),
             ),
             const SizedBox(height: 24),
-            CircularProgressIndicator(color: colorScheme.primary),
+            SizedBox(
+              width: 22,
+              height: 22,
+              child: CircularProgressIndicator(
+                strokeWidth: 2.5,
+                color: colorScheme.primary,
+              ),
+            ),
           ],
         ),
       ),