feat: Quick Actions panel (restart gateway, clear cache, rotate logs, health check)

[vivekchand]

Closes #252

What

Adds a new Actions tab to the ClawMetry dashboard with a Quick Actions panel for common operational tasks.

How

Backend (dashboard.py)

• POST /api/actions/run — executes one of 4 actions with a confirmation gate:
  • restart-gateway: tries openclaw gateway restart → systemctl --user restart openclaw → systemctl restart openclaw in order
  • clear-cache: purges ~/.clawmetry/cache, /tmp/clawmetry_cache, and resets the in-process OTEL store
  • rotate-logs: sends SIGUSR1 to openclaw/clawmetry processes + runs logrotate -f if available
  • health-check: gateway ping, disk usage, session count — returns a one-line summary
• GET /api/actions/history — returns in-memory ring-buffer of last 50 action results
• Both endpoints registered on the existing bp_health blueprint

Frontend (both dark + light themes)

• Actions nav tab added to both theme variants
• page-actions div with a 2-column card grid (icon + description + button per action)
• Confirmation dialog modal before executing any destructive action (restart, clear, rotate)
• Result banner (green on success, red on failure) with output and duration_ms
• Recent Actions table showing last 50 runs

Tests

• 5 new tests in TestQuickActions: POST-only enforcement, unknown action rejection, health-check, clear-cache, history structure
• All 81 existing tests pass, 6 skipped (unchanged)