feat(desktop): add session comparison

[justn-hyeok]

Problem

Users cannot compare two review sessions side-by-side.

Source

docs/DESKTOP_APP_CONSOLIDATION.md lists compare sessions as a dashboard capability to absorb.

Acceptance Criteria

• User can select two sessions and compare verdict, severity counts, findings, and cost/degraded signals.
• Works with filtered/sorted session lists.
• Empty/incompatible data is handled gracefully.
• Typecheck and smoke pass.