fix(bundle): ship spaceshd inside the .app (packaged GUI was offline)

tauri build bundled only the GUI binary, so the packaged app had no daemon:
find_daemon() looks for a sibling `spaceshd` next to the GUI
(Contents/MacOS/spaceshd) and ensure_daemon failed to spawn it → "offline".
(Dev worked only because find_daemon falls back to the repo-root target path.)

- tauri.bundle.conf.json: a build-only overlay adding bundle.externalBin
  ["bin/spaceshd"], kept out of tauri.conf.json so `tauri dev` doesn't require
  a sidecar file.
- Makefile: `make dmg` now builds spaceshd for both arches, lipo's a universal
  sidecar into src-tauri/bin/spaceshd-universal-apple-darwin, and passes
  --config so it lands in Contents/MacOS/spaceshd. `make dmg-native` does the
  host-arch equivalent.
- .gitignore: ignore the generated app/src-tauri/bin/.

After install, the unsigned helper runs once quarantine is cleared recursively:
xattr -dr com.apple.quarantine /Applications/spacesh.app

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src-tauri/tauri.bundle.conf.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "https://schema.tauri.app/config/2",
+  "bundle": {
+    "externalBin": ["bin/spaceshd"]
+  }
+}