feat: add desktop notification bridge

[rogerchappel]

Summary

• add desktop notification bridge methods for permission, requestPermission, and show
• validate notification payloads and keep routes constrained to CrewCmd paths
• focus the shell and navigate to the supplied CrewCmd route when a notification is clicked

Stack

• Base PR: chore: scaffold desktop shell #275
• Related docs PRs: docs: add desktop release checklist #277, docs: define desktop gateway discovery #278

Verification

• git diff --check
• node --check apps/desktop/src/main.cjs && node --check apps/desktop/src/preload.cjs
• cd apps/desktop && pnpm install --frozen-lockfile
• cd apps/desktop && pnpm check
• pnpm exec tsc --noEmit --pretty false
• push hook: pnpm typecheck
• push hook: pnpm build

Risk

Medium. Adds the first native desktop value, but does not add server event subscriptions, persistent permissions, secure storage, tray, updater, signing, or OpenClaw runtime bundling.