docs(readme): add macOS dmg download link and apps/desktop layout

Document Baidu Netdisk distribution for Apple Silicon builds and expand
desktop build/output instructions for contributors.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: heqikai

README.md

@@ -211,9 +211,21 @@ Roadmap and deferred work: [docs/codedelta/ROADMAP.md](docs/codedelta/ROADMAP.md
 
 ## Desktop (macOS)
 
-CodeDelta ships a **macOS desktop app** (`apps/desktop`) that bundles Node 20 and the API server — no separate Node install required for end users.
+CodeDelta ships a **macOS desktop app** ([`apps/desktop/`](apps/desktop/)) — a Tauri 2 shell that bundles Node 20 and the API server. End users do not need a separate Node install.
 
-**Requirements to build:** macOS (arm64 or x64), [Xcode Command Line Tools](https://developer.apple.com/xcode/resources/), [Rust 1.88+](https://rustup.rs/) (use `rustup`; Homebrew `cargo` alone may be too old), and repo dev dependencies (`npm ci`).
+### Download (Apple Silicon)
+
+Pre-built **unsigned** `.dmg` (arm64 / M1–M4):
+
+- [百度网盘](https://pan.baidu.com/s/16DL3nvpFA9FR6UsskqoFyA?pwd=frog) · 提取码: `frog`
+
+Install: open the dmg → drag **CodeDelta** to Applications. If macOS blocks launch, right-click the app → **Open**. Requires **git** on `PATH`.
+
+**Runtime data:** `~/Library/Application Support/CodeDelta` (repos, snapshots, settings).
+
+### Build from source
+
+**Requirements:** macOS (arm64 or x64), [Xcode Command Line Tools](https://developer.apple.com/xcode/resources/), [Rust 1.88+](https://rustup.rs/) via `rustup` (Homebrew `cargo` alone may be too old), and repo dev dependencies (`npm ci`).
 
 ```bash
 # One-time: stage embedded Node + server runtime (~200MB under apps/desktop/src-tauri/resources/runtime/)
@@ -226,9 +238,20 @@ npm run build:desktop
 npm run dev:desktop
 ```
 
-**Runtime data:** `~/Library/Application Support/CodeDelta` (repos, snapshots, settings).
+Output: `apps/desktop/src-tauri/target/release/bundle/dmg/CodeDelta_*_aarch64.dmg` (or copy to `release/` manually if `bundle_dmg.sh` fails).
+
+**`apps/desktop/` layout:**
+
+```
+apps/desktop/
+  package.json              # tauri dev / build:app scripts
+  src-tauri/
+    tauri.conf.json         # window, bundle, embedded resources
+    src/server.rs           # spawn bundled Node API on launch
+    resources/runtime/      # generated by npm run stage:desktop (gitignored)
+```
 
-**Git:** must be on `PATH` for import/compare/trace (the app shows a banner if `git` is missing).
+**Git:** the app shows a banner if `git` is missing.
 
 | Variable | Desktop default | Meaning |
 |----------|-----------------|---------|