Remove Gemini OAuth support (#698)

* remove Gemini OAuth support

Google Gemini OAuth adds complexity without sufficient value — users
can use API keys directly. Removes the provider implementation,
registration, mapping, UI fallback, tests, and documentation references.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: ignore leftover OAuth config for providers without OAuth mapping

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: remove leftover Gemini reference in useOAuth comment

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
(cherry picked from commit c1705d9670c9a2bca7a5f499e2caeed19589a934)