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/keychain.ts

@@ -0,0 +1,17 @@
+import { saveConnectionPassword, deleteConnectionPassword } from "./commands";
+
+// Persist (or purge) the DB password according to the keychain toggle.
+// use_keychain defaults to true (opt-out): ON = OS keychain; OFF = purge + session-only.
+// Shared by connectionStore.createConnection and EditConnectionModal.
+export async function persistDbPassword(
+  connectionId: string,
+  useKeychain: boolean | undefined,
+  password: string | null | undefined,
+): Promise<void> {
+  const useKc = useKeychain ?? true;
+  if (useKc && password) {
+    await saveConnectionPassword(connectionId, password);
+  } else if (!useKc) {
+    try { await deleteConnectionPassword(connectionId); } catch { /* purge; ignore missing */ }
+  }
+}