Add desktop search and result anchors

[wesbillman]

Summary

• add relay-backed desktop search with a Cmd+K dialog and replace the old sidebar jump input
• wire relay and Tauri event lookup so opening a search result lands on the matched message in the right channel
• extend the desktop e2e bridge and smoke coverage for search result navigation

Testing

• cargo check --workspace
• cd desktop && pnpm check
• cd desktop && pnpm typecheck
• cd desktop && pnpm build
• cd desktop && pnpm exec playwright test --project=smoke --grep "loads the exact result"
• pre-push hook (desktop-check, desktop-build, desktop-tauri-check, rust-fmt, rust-clippy, rust-tests)

[wesbillman]