refactor(ui): centralize event subscriptions

[slkzgm]

This is PR1 from the RFC/design discussion in #54.

What

UI-only refactor (no intended behavior changes).

• Add src/services/events.ts to centralize event subscriptions.
• Update:
  • src/features/app/hooks/useAppServerEvents.ts
  • src/features/terminal/hooks/useTerminalSession.ts
    to use the new subscription helpers.

Why

For an optional "remote backend" mode (e.g. WSL2 daemon + native Windows UI), we need the UI to be able to swap the event source (Tauri listen(...) today, WebSocket tomorrow) without rewriting multiple hooks/components.

This PR moves the event wiring behind a small service module so later work can introduce alternate implementations in one place.

Notes

• Event names/payloads remain unchanged (app-server-event, terminal-output).
• No UI/UX changes expected.

Validation

• npm ci
• npm run typecheck
• npm run build

[slkzgm]

Rebased onto latest main to account for the recent frontend re-org (hooks moved under src/features/*).

The branch should now be mergeable cleanly.