feat(ui): overhaul dashboard with theme-aware charts, auto-refresh, and system metrics

[geoffjay]

• Replace hardcoded chart hexes with a theme-derived palette (useChartPalette):
  muted alpha fills, full-strength colors only for legends/readouts, softened
  grid and tick lines so charts match the active theme
• Add shared usePolling (30s, pauses on hidden tab) and apply to all dashboard
  data hooks; Refresh button now refreshes every source with an updated-at stamp
• Add MonitorClient and a CPU/memory time-series chart fed by monitor /history;
  monitor joins the service health grid (6 services)
• Add OverviewStats row (agents, approvals, notifications, questions, workflows,
  cost) with graceful degradation per source
• Add stacked 24h ActivityChart and make the recent-activity feed show real
  notifications, questions, and agent state changes via useActivityFeed
• Fix MSW handler ordering bug (/notifications/:id shadowing count/actionable/history)
• Document API instrumentation gaps in docs/instrumentation.md

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

[geoffjay]

This change is part of the following stack:

• feat(ui): overhaul dashboard with theme-aware charts, auto-refresh, and system metrics #1255 ◀

_{Change managed by git-spice.}

[codecov]

Codecov Report

❌ Patch coverage is 83.01887% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.88%. Comparing base (e02408c) to head (c8e5844).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
ui/src/components/dashboard/SystemMetricsChart.tsx	46.87%	16 Missing and 1 partial ⚠️
ui/src/pages/DashboardPage.tsx	46.66%	16 Missing ⚠️
ui/src/components/dashboard/ActivityChart.tsx	70.83%	7 Missing ⚠️
ui/src/services/monitor.ts	63.63%	4 Missing ⚠️
...i/src/components/dashboard/NotificationSummary.tsx	57.14%	3 Missing ⚠️
ui/src/hooks/useActivityFeed.ts	96.87%	2 Missing ⚠️
ui/src/hooks/useChartPalette.ts	93.10%	2 Missing ⚠️
ui/src/components/dashboard/AgentSummary.tsx	75.00%	1 Missing ⚠️
ui/src/hooks/useDashboardStats.ts	95.23%	1 Missing ⚠️
ui/src/hooks/useSystemMetrics.ts	97.14%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1255      +/-   ##
==========================================
+ Coverage   45.45%   45.88%   +0.43%     
==========================================
  Files         376      385       +9     
  Lines       26755    27042     +287     
  Branches     2368     2451      +83     
==========================================
+ Hits        12161    12409     +248     
- Misses      14573    14611      +38     
- Partials       21       22       +1     

Flag	Coverage Δ
frontend	68.18% <83.01%> (+0.75%)	⬆️

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