feat(desktop): add diff preview before running review

[justn-hyeok]

Problem

The desktop app can launch staged/working-tree reviews, but it does not show a pre-flight diff preview or allow users to inspect what will be reviewed.

Source

docs/DESKTOP_APP_CONSOLIDATION.md lists staged/unstaged diff selection as a TUI capability the desktop should absorb.

Acceptance Criteria

• Run Review view shows staged and/or working-tree diff preview before launch.
• User can distinguish staged vs unstaged inputs.
• Empty/no-diff state is clear and actionable.
• Typecheck and smoke pass.