feat: cost alerting & budget burn rate widget

[vivekchand]

Closes #200

What

Surfaces the budget alerting system that was already built but hidden behind commented-out UI, and adds a new burn rate widget to the overview dashboard.

How

New /api/budget/burn-rate endpoint

• Returns 7-day daily cost history (labelled Mon/Tue/etc)
• Calculates daily average from non-zero days (avoids skewing on new installs)
• Returns projected monthly cost (daily_avg * 30)
• Includes budget limit headroom (daily_pct, monthly_pct)
• Includes pause state so UI can show paused banner

Budget & Alerts button un-hidden

• The openBudgetModal() button was commented out in both light and dark nav themes with a 'hidden until mature' note — it is mature now
• Users can now access daily/weekly/monthly budget limits, auto-pause, alert rules, and Telegram notifications directly from the nav

Burn Rate widget on overview

• Compact panel added to the overview right pane (below Active Tasks)
• Shows: today's spend, 7-day daily average, projected monthly cost
• Color-coded projected cost (green < $5, amber < $20, red >= $20)
• Budget progress bars (shown only when limits are configured)
• 7-day mini sparkline (current day highlighted in accent color)
• Paused banner when auto-pause has triggered
• Widget is hidden when there is no spend data (zero-cost / OAuth users)
• Configure button opens the Budget & Alerts modal

Tests

6 new tests added — all 139 tests pass (+ 6 skipped E2E).