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.test.ts

@@ -0,0 +1,23 @@
+import { describe, it, expect, vi, afterEach } from "vitest";
+import { isMacOS } from "./platform";
+
+describe("isMacOS", () => {
+  afterEach(() => {
+    vi.stubGlobal("navigator", undefined);
+  });
+
+  it("true on MacIntel/Mac platform", () => {
+    vi.stubGlobal("navigator", { platform: "MacIntel", userAgent: "Mozilla/5.0 (Macintosh; X)" });
+    expect(isMacOS()).toBe(true);
+  });
+
+  it("false on Win32", () => {
+    vi.stubGlobal("navigator", { platform: "Win32", userAgent: "Mozilla/5.0 (Windows NT 10.0)" });
+    expect(isMacOS()).toBe(false);
+  });
+
+  it("false on Linux", () => {
+    vi.stubGlobal("navigator", { platform: "Linux x86_64", userAgent: "Mozilla/5.0 (X11; Linux)" });
+    expect(isMacOS()).toBe(false);
+  });
+});