refactor: convert to bun-workspaces monorepo (desktop/ + www/)

- Move the Tauri app (React frontend + Rust backend) into desktop/ via
  git mv — configs unchanged (relative paths: tauri.conf.json, vite.config.ts)
- Root package.json becomes a private bun workspace container with
  orchestration scripts; desktop package renamed gridline-desktop
- Scaffold www/ with Astro 7 + Tailwind v4 (hand-wired vite plugin,
  static output, ready for Dokploy)
- Update release.yml for the new layout: projectPath: desktop,
  desktop/src-tauri resource paths, rust-cache workspace path
- Update README + AGENTS.md structure trees and dev commands
- Verified: vitest (1074), cargo test (354), desktop build, tauri dev
  (window launches), tauri build (dmg + app bundles), Astro build

desktop/bunfig.toml

@@ -0,0 +1,5 @@
+# Bun test runner configuration. `bun test` uses Bun's native runner (it does
+# not read vite.config.ts), so the jsdom/DOM preload lives here. The canonical
+# test command is `bun run test` (vitest run), which uses vite.config.ts.
+[test]
+preload = ["./src/test/bun-dom.ts", "./src/test/bun-setup.ts"]