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/widgets/profile_avatar_button.dart

@@ -1,14 +1,37 @@
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'package:provider/provider.dart';
 import '../providers/server_provider.dart';
 import '../views/account_view.dart';
 
 /// Top-bar avatar button that opens the account screen. Replaces the old
 /// "Account" bottom-nav destination, matching how Google Drive surfaces
-/// account access from a profile picture in the app bar.
+/// account access from a profile picture in the app bar. Also doubles as a
+/// quick account switcher: swiping up/down on it cycles to the next/
+/// previous saved account immediately, with no confirmation - a shortcut
+/// alongside the full switcher list in Settings.
 class ProfileAvatarButton extends StatelessWidget {
   const ProfileAvatarButton({super.key});
 
+  void _handleVerticalSwipe(BuildContext context, DragEndDetails details) {
+    final velocity = details.primaryVelocity ?? 0;
+    if (velocity.abs() < 250) return;
+
+    final provider = context.read<ServerProvider>();
+    final target = velocity < 0
+        ? provider.cycleToNextAccount()
+        : provider.cycleToPreviousAccount();
+    if (target == null) return;
+
+    HapticFeedback.selectionClick();
+    ScaffoldMessenger.of(context).showSnackBar(
+      SnackBar(
+        content: Text('Switched to ${target.username}'),
+        behavior: SnackBarBehavior.floating,
+      ),
+    );
+  }
+
   @override
   Widget build(BuildContext context) {
     final provider = context.watch<ServerProvider>();
@@ -19,23 +42,26 @@ class ProfileAvatarButton extends StatelessWidget {
 
     return Padding(
       padding: const EdgeInsets.only(right: 8),
-      child: IconButton(
-        tooltip: 'Settings',
-        onPressed: () {
-          Navigator.push(
-            context,
-            MaterialPageRoute(builder: (_) => const AccountView()),
-          );
-        },
-        icon: CircleAvatar(
-          radius: 17,
-          backgroundColor: colorScheme.primary,
-          child: Text(
-            initial,
-            style: TextStyle(
-              color: colorScheme.onPrimary,
-              fontWeight: FontWeight.bold,
-              fontSize: 14,
+      child: GestureDetector(
+        onVerticalDragEnd: (details) => _handleVerticalSwipe(context, details),
+        child: IconButton(
+          tooltip: 'Settings',
+          onPressed: () {
+            Navigator.push(
+              context,
+              MaterialPageRoute(builder: (_) => const AccountView()),
+            );
+          },
+          icon: CircleAvatar(
+            radius: 17,
+            backgroundColor: colorScheme.primary,
+            child: Text(
+              initial,
+              style: TextStyle(
+                color: colorScheme.onPrimary,
+                fontWeight: FontWeight.bold,
+                fontSize: 14,
+              ),
             ),
           ),
         ),