AppForge: migrate off Supabase to external Postgres + Better Auth + local storage

- New Express + TypeScript backend (server/) with pg, Better Auth, local file storage
- De-Supabased Postgres schema (server/db) and TS reimplementations of DB functions
- Frontend data layer rewired to REST (rest-client + backend-client compat shim)
- Removed all Supabase references (code, config, deps, docs)
- New brand assets: gradient favicon/app icons + dark/white wordmark logos

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/favicon.svg

@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" width="512" height="512" role="img" aria-label="AppForge">
+  <defs>
+    <linearGradient id="afg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#8B5CF6"/>
+      <stop offset="0.55" stop-color="#6366F1"/>
+      <stop offset="1" stop-color="#0EA5E9"/>
+    </linearGradient>
+  </defs>
+  <rect x="32" y="32" width="448" height="448" rx="112" fill="url(#afg)"/>
+  <path d="M292 80 L150 296 H238 L220 432 L372 208 H280 Z"
+        fill="#ffffff" stroke="#ffffff" stroke-width="6" stroke-linejoin="round"/>
+</svg>