feat(desktop): add model leaderboard and cost analytics views

[justn-hyeok]

Problem

Desktop shows only per-session cost summary. It lacks aggregate model/cost analytics.

Source

docs/DESKTOP_APP_CONSOLIDATION.md lists model leaderboard and cost analytics.

Acceptance Criteria

• Add a read-only analytics surface for model/provider usage where session telemetry exists.
• Include cost totals/trends and model/provider breakdowns.
• Missing or unknown costs are clearly labeled.
• Typecheck and smoke pass.