feat: /cost TUI slash command with usage dialog

[e6qu]

Summary

• Add /cost (alias /usage) TUI-only slash command that opens a compact usage dialog showing session, daily, and monthly API cost metrics with cache hit rates
• Add GET /session/stats server endpoint reusing existing aggregateSessionStats logic
• Extract pure cost-metrics module (cost-metrics.ts) for testability
• Add 25 unit tests + 4 UI tests (first usage of @opentui/solid testRender in the codebase)

Dialog output

┌─ Usage ───────────────────┐
│ Sess  $0.4╱$1.9     ⟐85% │
│ ☼-ly  $12.3╱$34.5   ⟐72% │
│ ☽-ly  $148╱$412     ⟐68% │
└───────────────────────────┘

Columns: actual/no-cache cost pair, cache hit %. Ephemeral — no messages sent to LLM.

Test plan

• 25 unit tests for formatCost, findModel, computeSessionMetrics, computeStatsMetrics, formatRow
• 4 UI tests rendering DialogCost via testRender + captureCharFrame with mocked contexts
• Full test suite passes (1376 tests, 0 failures)
• Manual: bun run dev → open session → /cost → verify dialog renders → press esc → dialog dismisses
• Manual: verify no messages added to session after /cost