Show channels in command search

[tlongwell-block]

Summary

• add channel name/description matches to the cmd+k search dialog
• open channel results directly while preserving existing message-hit navigation
• cover channel search with a smoke e2e test

Test plan

• pnpm --dir desktop typecheck
• pnpm --dir desktop exec biome check src/features/search/ui/SearchDialog.tsx src/app/AppShellOverlays.tsx tests/e2e/smoke.spec.ts
• pnpm --dir desktop exec playwright test --project=smoke tests/e2e/smoke.spec.ts --grep "opens channel matches from search|opens relay-backed search"
• pre-commit hook: desktop-check, desktop-tauri-fmt, mobile-check, rust-fmt, web-check
• pre-push hook: web-check, rust-fmt, desktop-tauri-fmt, desktop-check, web-build, mobile-check, rust-clippy, desktop-build, mobile-test, rust-tests, desktop-tauri-check