feat: grammar API structured output + Android waitlist link fix #4750

Originally written by @ccantynz-alt on GitHub.
Imported from https://github.com/ccantynz-alt/voxlen/pull/51

Summary

• Grammar API now returns changes[] and score — previously the proxy returned only { corrected }, so the desktop app's Grammar panel showed no change details (empty diff view). System prompt updated to request structured JSON; response parser handles fallback if Claude returns plain text.
• Android platform card links to waitlist form — "Join waitlist" link was pointing to #download (the download section), which scrolled past the actual Android waitlist form. Added id="android-waitlist" to the form container and fixed the href.

Test plan

• Grammar correction via Voxlen account key shows change details in the Grammar panel
• Clicking "Join waitlist" on the Android platform card scrolls to the Android waitlist form
• Grammar fallback: if Claude returns plain text (not JSON), correction still works

https://claude.ai/code/session_011MFNHeFKrhfuSrEYVgrdyr

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