feat: add google_only bootstrap mode (#62)

Second operating mode for users whose network already blocks
script.google.com and therefore cannot reach it to deploy Code.gs
in the first place. In google_only, the client runs only the
SNI-rewrite tunnel to *.google.com and the other Google-edge
suffixes that are already allowlisted; non-Google traffic falls
through to direct TCP. No script_id or auth_key is required. Once
Code.gs is deployed, the user switches to apps_script mode and
pastes the Deployment ID.

- config: Mode enum, relaxed validation when mode is google_only
- proxy_server: mode check in dispatch_tunnel; DomainFronter is now
  Option<Arc<_>> so it is not constructed in google_only
- desktop UI and Android app: Mode dropdown, Apps Script fields
  disable in google_only
- README: bootstrap subsection in English and Persian
- config.google-only.example.json
- version bump to 1.2.0 + changelog entry

Backward compatible with existing apps_script configs.

android/app/build.gradle.kts

@@ -14,8 +14,8 @@ android {
         applicationId = "com.therealaleph.mhrv"
         minSdk = 24 // Android 7.0 — covers 99%+ of live devices.
         targetSdk = 34
-        versionCode = 115
-        versionName = "1.1.5"
+        versionCode = 120
+        versionName = "1.2.0"
 
         // Ship all four mainstream Android ABIs:
         //   - arm64-v8a      — 95%+ of real-world Android phones since 2019