feat: support Antigravity CLI and update default Gemini models to v3 (Gemini 3.5)

[HarithSami]

Describe the Feature Request

1. Antigravity CLI Support: The gemini_cli provider is now deprecated. It should be replaced/updated with support for the antigravity_cli to ensure keyless local Google model integration continues working.
2. Default Models Update: The default Gemini models inside the LLM model picker should be bumped from the 2nd generation to the 3rd generation (e.g., defaulting to gemini-3.5-flash and gemini-3.5-pro for faster and more accurate resume scoring/generation).

Proposed Changes

• Update LLM subscription provider logic to call antigravity instead of gemini in CLI wrappers.
• Refresh default model configurations in backend/core/config.py to point to gemini-3.5 variants by default.

[vasu-devs]

Implemented on dev (commit 9548219): antigravity_cli is a first-class subscription provider (headless JSON exec, auth detection for ~/.antigravity with ~/.gemini fallback, install hint, settings + model picker with gemini-3.5-pro/flash), and the keyed Gemini default is bumped to gemini-3.5-flash.

One ask: the Antigravity CLI's headless flags are implemented against the same contract as gemini-cli (--output-format json, -m, prompt on stdin). If your installed antigravity binary uses different flags, drop them here and I'll adjust before release.