Restructure into pnpm monorepo with game shell, docs, and SpacetimeDB backend

- Restructure flat static prototype into pnpm workspace monorepo
- apps/game: playable shell with R3F 3D scene, HUD, SpacetimeDB connection
- apps/docs: design docs and prototypes
- apps/site: landing page
- packages/ui: shared Button and Panel primitives
- services/spacetimedb: backend module (9 tables, 11 reducers)
- Archive legacy static files to archive/legacy-static/
- Game loop: connect, undock, target, approach, dock, mine, sell
- Add pnpm-workspace.yaml, tsconfig.base.json, spacetime.json

apps/docs/src/components/NotFound.tsx

@@ -0,0 +1,11 @@
+import { Link } from "react-router-dom";
+
+export function NotFound() {
+  return (
+    <div className="mx-auto max-w-content">
+      <h1>Page Not Found</h1>
+      <p className="text-fg-dim">The requested route does not exist.</p>
+      <Link className="inline-flex items-center gap-2 rounded-lg border border-accent bg-accent px-4 py-2 text-[0.8rem] font-semibold text-bg transition-all duration-150 hover:bg-accent-hover" to="/docs">Back to documentation</Link>
+    </div>
+  );
+}