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/src/lib/platform.ts

@@ -0,0 +1,9 @@
+/** True only on macOS (the macOS "Overlay" drag strip is gated on this). Uses
+ * navigator.platform/userAgent (the established pattern in BackupPage.tsx);
+ * @tauri-apps/plugin-os is not installed and getCurrentWindow() has no osLabel(). */
+export function isMacOS(): boolean {
+  if (typeof navigator === "undefined") return false;
+  const p = (navigator.platform || "").toLowerCase();
+  const u = (navigator.userAgent || "").toLowerCase();
+  return p.includes("mac") || u.includes("macintosh") || u.includes("mac os");
+}