chore(release): v1.4.0-beta.2

Bump 1.4.0-beta.1 -> 1.4.0-beta.2 across package.json, tauri.conf.json,
and Cargo.toml/Cargo.lock.

Beta update channel:
- github-releases: channel-aware fetching + pre-release-aware semver
  (getLatestRelease, semverCompare), stable-only entitlement preserved
- use-app-updater: when beta opt-in is on, surface the newest pre-release
  via a manual (release-page) update notice; stable silent updater and the
  license entitlement window are untouched
- SettingsPage: beta update card + toggling "Receive beta updates" re-checks
  immediately

Also bundles in-progress local-embedding / semantic-search work.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jiaweing

apps/desktop/package.json

@@ -1,7 +1,7 @@
 {
   "name": "desktop",
   "private": true,
-  "version": "1.4.0-beta.1",
+  "version": "1.4.0-beta.2",
   "type": "module",
   "scripts": {
     "dev": "tauri dev -- --features bria",