Desktop app should resume existing sessions when reopened

[anduimagui]

Problem

The desktop app mounts packages/app, but the app session page only syncs the session data when reopening an existing session. It does not call the existing session resume lifecycle that was added on the backend and already used by the CLI/TUI.

Expected behavior

Reopening an existing session in the desktop app should trigger the resume lifecycle once, while brand-new sessions and freshly forked sessions should skip their initial resume call.

Proposed fix

Port the app-side resume behavior from the TUI into packages/app, reuse the shared resume helper logic for the session page, and add coverage for the one-shot skip behavior for new sessions and forks.

Related context

This depends on the backend/session lifecycle work in #18007.

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