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/components/TokenUserSelect.tsx

@@ -0,0 +1,38 @@
+"use client";
+
+import { useOptimistic, useTransition } from "react";
+
+interface TokenUserSelectProps {
+  tokenId: string;
+  currentUserId?: string;
+  users: { id: string; username: string }[];
+  action: (formData: FormData) => Promise<void>;
+  noUserLabel: string;
+}
+
+export function TokenUserSelect({ tokenId, currentUserId, users, action, noUserLabel }: TokenUserSelectProps) {
+  const [optimisticValue, setOptimisticValue] = useOptimistic(currentUserId ?? "");
+  const [, startTransition] = useTransition();
+
+  return (
+    <select
+      value={optimisticValue}
+      onChange={(e) => {
+        const newValue = e.target.value;
+        startTransition(async () => {
+          setOptimisticValue(newValue);
+          const fd = new FormData();
+          fd.append("id", tokenId);
+          fd.append("user_id", newValue);
+          await action(fd);
+        });
+      }}
+      className="flex h-8 rounded-md border border-input bg-background px-2 py-0 text-xs shadow-sm transition-colors focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 disabled:cursor-not-allowed disabled:opacity-50"
+    >
+      <option value="">{noUserLabel}</option>
+      {users.map((u) => (
+        <option key={u.id} value={u.id}>{u.username}</option>
+      ))}
+    </select>
+  );
+}