feat: multi-user reading progress & backoffice impersonation

- Scope all reading progress (books, series, stats) by user via
  Option<Extension<AuthUser>> — admin sees aggregate, read token sees own data
- Fix duplicate book rows when admin views lists (IS NOT NULL guard on JOIN)
- Add X-As-User header support: admin can impersonate any user from backoffice
- UserSwitcher dropdown in nav header (persisted via as_user_id cookie)
- Per-user filter pills on "Currently reading" and "Recently read" dashboard sections
- Inline username editing (UsernameEdit component with optimistic update)
- PATCH /admin/users/:id endpoint to rename a user
- Unassigned read tokens row in users table
- Komga sync now requires a user_id — reading progress attributed to selected user
- Migration 0051: add user_id column to komga_sync_reports
- Nav breakpoints: icons-only from md, labels from xl, hamburger until md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/backoffice/app/(app)/settings/page.tsx

@@ -1,4 +1,4 @@
-import { getSettings, getCacheStats, getThumbnailStats } from "@/lib/api";
+import { getSettings, getCacheStats, getThumbnailStats, fetchUsers } from "@/lib/api";
 import SettingsPage from "./SettingsPage";
 
 export const dynamic = "force-dynamic";
@@ -23,5 +23,7 @@ export default async function SettingsPageWrapper() {
     directory: "/data/thumbnails"
   }));
 
-  return <SettingsPage initialSettings={settings} initialCacheStats={cacheStats} initialThumbnailStats={thumbnailStats} />;
+  const users = await fetchUsers().catch(() => []);
+
+  return <SettingsPage initialSettings={settings} initialCacheStats={cacheStats} initialThumbnailStats={thumbnailStats} users={users} />;
 }