feat(viewer): add anchored surface comments

[benvinegar]

Summary

• Add anchored comment metadata to comments, including SQLite/JSON store persistence and sanitized REST input.
• Add host-owned avatar-pin annotations over surfaces, with hover/focus popovers, inline composer, and delete support.
• Include anchor context in feedback/MCP responses and cover anchor/deletion flows in tests.

Validation

• npm run typecheck
• npm run lint
• npm run format:check
• npm test
• npm run build:viewer

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