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

www/AGENTS.md

@@ -0,0 +1,22 @@
+## Development
+
+When starting the dev server, use background mode:
+
+```
+astro dev --background
+```
+
+Manage the background server with `astro dev stop`, `astro dev status`, and `astro dev logs`.
+
+## Documentation
+
+Full documentation: https://docs.astro.build
+
+Consult these guides before working on related tasks:
+
+- [Adding pages, dynamic routes, or middleware](https://docs.astro.build/en/guides/routing/)
+- [Working with Astro components](https://docs.astro.build/en/basics/astro-components/)
+- [Using React, Vue, Svelte, or other framework components](https://docs.astro.build/en/guides/framework-components/)
+- [Adding or managing content](https://docs.astro.build/en/guides/content-collections/)
+- [Adding styles or using Tailwind](https://docs.astro.build/en/guides/styling/)
+- [Supporting multiple languages](https://docs.astro.build/en/guides/internationalization/)