fix: resolve React Error #310 and refactor admin section

- Fix React bug #33580: remove Suspense boundaries co-located with Link components
- Delete settings/loading.tsx and admin/loading.tsx (root cause of race condition)
- Convert all admin navigation from Next.js Link to anchor tags
- Move admin pages to dedicated (admin) route group
- Add AdminHeader matching main header visual design
- Add AdminSidebar with anchor-based navigation
- Add /api/admin/models route handler (replaces server actions for GET)
- Add /api/debug/client-error for server-side browser error reporting
- Add useNoteRefreshOptional() to fix crash in AdminHeader
- Hide Admin Dashboard menu for non-admin users
- Change app icons from yellow to blue (#3A7CA5) matching brand primary
- Fix admin search bar width to match main header

Made-with: Cursor

memento-note/public/icons/icon-192.svg

@@ -1,4 +1,10 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 192 192">
-  <rect width="192" height="192" fill="#f59e0b" rx="40"/>
-  <path d="M50 50 h92 v92 h-92 z" fill="#fff"/>
-</svg>
+  <rect width="192" height="192" fill="#3A7CA5" rx="40"/>
+  <g transform="translate(96,96)">
+    <rect x="-34" y="-34" width="68" height="68" rx="5" fill="white"/>
+    <path d="M34,34 L34,11 L11,34 Z" fill="#3A7CA5" opacity="0.3"/>
+    <line x1="-23" y1="-17" x2="23" y2="-17" stroke="#e5e7eb" stroke-width="3" stroke-linecap="round"/>
+    <line x1="-23" y1="-4" x2="23" y2="-4" stroke="#e5e7eb" stroke-width="3" stroke-linecap="round"/>
+    <line x1="-23" y1="9" x2="11" y2="9" stroke="#e5e7eb" stroke-width="3" stroke-linecap="round"/>
+  </g>
+</svg>