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/index.ts

@@ -0,0 +1,4 @@
+export { Button } from "./primitives/Button";
+export type { ButtonProps } from "./primitives/Button";
+export { Panel } from "./primitives/Panel";
+export type { PanelProps } from "./primitives/Panel";