Show context cache hit rate with decimals / 使用小数显示上下文缓存命中率

[SivanCola]

Summary

• Show the right-side context panel cache-hit metric with two decimal places instead of integer rounding.
• Keep the existing low-cache health behavior by rounding only for the health threshold.
• Add targeted coverage for 99.95%, 0.00%, and the no-usage - state.

Verification

• git diff --check
• npm run check:css
• npx tsx src/__tests__/context-panel-breakdown.test.ts
• Not run: npm run typecheck (blocked because generated Wails bindings are not present in this checkout: wailsjs/go/main/App, wailsjs/runtime/runtime)

Related

• Adjacent open PRs touch ContextPanel cost/status metrics ([codex] Show status bar amounts in RMB #4485, Integrate clear context telemetry fixes / 集成清空上下文与统计面板修复 #4329), but this PR is limited to cache-hit rate formatting.