fix: restore backtest results experience

[pjy008008]

What changed

• Reorganized completed backtest results into performance, monthly analysis, trades, and execution tabs.
• Restored a readable performance summary with metric help tooltips.
• Simplified monthly analysis, added explanations for all monthly counters and first-failure conditions, and removed internal identifiers and timezone noise.
• Fixed selected-control contrast in both light and dark themes.
• Added unit and browser coverage for the restored interactions and theme behavior.

Why

Real backtest data made the earlier prototype layout dense and exposed implementation details. This change restores a simple customer-facing review flow while continuing to render the existing API contract.

User impact

Customers can scan results by category, understand each metric through hover or keyboard focus, and review monthly and trade details without internal execution keys.

Checks

• pnpm vitest run src/BacktestLiveView.test.tsx src/BacktestView.test.tsx src/EnglishLocale.test.tsx (68 passed)
• pnpm exec playwright test e2e/backtest.e2e.ts (8 passed)
• pnpm typecheck
• pnpm build