Add layout error boundaries for dashboard

[Soengkit]

Summary

Adds reusable React error boundaries around the dashboard and key layout sections so render failures in dynamic UI areas do not blank the whole app.

Addresses #1.

Changes

• Adds frontend/src/components/ErrorBoundary.tsx with typed React error-boundary state and an accessible design-token-aligned fallback UI.
• Wraps Header, Sidebar, and routed page content in layout-level boundaries.
• Splits dashboard stat rendering into StatCard and StatCardsGrid, then wraps the dashboard panel and stat grid in distinct boundaries.
• Removes the dashboard stat any cast by typing stat-card keys against DashboardStats.

Testing

• npm run build from frontend/ - passed (tsc -b && vite build).
• git diff --check - passed.
• python3 build.py - ran and committed diagnostic/build-1702a6da.json plus diagnostic/build-1702a6da.logd. The script returned non-zero because 3 unrelated non-frontend modules failed while diagnostics were still produced:
  • frailbox: existing C compile warning/error in src/arena.c.
  • compliance: local runner could not locate a Java Runtime.
  • openapi-haskell: local GHC environment is missing Data.Yaml.
• Diagnostic log password: 4b7a5a8ab0315b237719.

Checklist

• Affected frontend module builds locally.
• Required diagnostic .json and .logd artifacts are committed.
• No generated build artifacts are committed, except the required diagnostic build log.
• No documentation, configuration, or schema changes are required for this frontend-only change.
• Changes are scoped to the PR purpose and avoid unrelated cleanup.
• Security, privacy, and error-handling implications have been considered.