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

packages/ui/src/primitives/Panel.tsx

@@ -0,0 +1,22 @@
+import type { HTMLAttributes, ReactNode } from "react";
+
+export type PanelProps = HTMLAttributes<HTMLElement> & {
+  as?: "article" | "section" | "div";
+  children: ReactNode;
+};
+
+export function Panel({ as: Tag = "section", children, className = "", ...props }: PanelProps) {
+  return (
+    <Tag
+      className={[
+        "rounded-lg border border-border bg-surface/92 p-5 shadow-[0_16px_60px_rgba(0,0,0,0.22)]",
+        className,
+      ]
+        .filter(Boolean)
+        .join(" ")}
+      {...props}
+    >
+      {children}
+    </Tag>
+  );
+}