fix(gemini): handle blob-backed generated images

[goneflyin]

Summary

• detect Gemini generated images that now render as blob: URLs, not only lh3 gg-dl URLs
• convert blob-backed images to base64 in the Gemini page context before saving
• keep existing gg-dl URL download path for older UI responses
• add a regression test for blob-backed image detection

Why

Gemini image generation can complete successfully in the browser but surf gemini --generate-image keeps polling until timeout because the UI now renders the generated image as blob:https://gemini.google.com/.... The existing poller only looks for img[src*="gg-dl"], so image-only responses have no success condition.

Test plan

• npm test -- --run test/unit/gemini-client.test.ts
• npm test -- --run test/unit/chatgpt-client.test.ts test/unit/grok-client.test.ts

Note: npm run check and npm run lint currently fail on main due to unrelated existing TypeScript/Biome config errors.

[goneflyin]

In all candor, totally used Pi to generate this PR without looking closely at the surf code... not my milieu.

Pretty sure it works and is accurate, but feel free to yell at me if not and I'll revise as needed. Thanks for the tool!