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.

docs/changelog/v1.2.0.md

@@ -0,0 +1,28 @@
+<!--
+Telegram changelog posted automatically by .github/workflows/release.yml
+on every tag push. Format:
+  - Persian changelog first (goes in an HTML <blockquote>)
+  - A separator line of just `---`
+  - English changelog second (also in a <blockquote>)
+  - Leave ASCII `-`, digits, and "issue #NN" refs as-is
+
+The workflow splits on the `---` separator. Bullets can be whatever
+you like; the bot forwards verbatim.
+-->
+
+• حالت جدید «فقط گوگل» (بوت‌استرپ): دسترسی مستقیم به *.google.com برای استقرار Code.gs وقتی هنوز به script.google.com دسترسی ندارید — بدون نیاز به Deployment ID یا Auth key
+
+• انتخابگر حالت در UI دسکتاپ و اندروید؛ CLI از طریق فیلد `mode` در config
+
+• سازگاری کامل با حالت apps_script؛ کانفیگ‌های موجود بدون تغییر بارگذاری می‌شوند
+
+• نمونه کانفیگ آمادهٔ google_only در ریشهٔ پروژه (`config.google-only.example.json`)
+
+---
+• New "Google-only" bootstrap mode: direct SNI-rewrite tunnel to *.google.com so users blocked from script.google.com can still reach it to deploy Code.gs. No Deployment ID or Auth key needed. Non-Google traffic goes direct.
+
+• Mode selector added to the desktop UI and the Android app; CLI picks it up from the `mode` field in config.
+
+• Fully backward compatible with apps_script mode — existing configs load unchanged.
+
+• New ready-to-use `config.google-only.example.json` at the repo root.