feat: improve media previews and message scrolling

[shakkernerd]

Summary

• Add upload media dimensions/duration metadata and persist it through SQLite, API responses, OpenAPI, and web types.
• Virtualize the main message list with scroll restoration, bottom pinning, and quote-jump support.
• Improve media preview sizing for images/videos using probed dimensions.
• Extract realtime connection/style entrypoints and thread empty state.
• Switch the web UI to Geist/Geist Mono and embed the rebuilt web assets exactly.

Verification

• pnpm build
• pnpm typecheck
• go test ./apps/api/internal/store/sqlite ./apps/api/internal/httpapi
• pnpm fmt:go:check
• pnpm fmt:ts:check
• pnpm lint
• diff -qr apps/web/dist apps/api/internal/webassets/dist
• git diff --check