Add dashboard error boundaries

[Florcitaq]

Closes #1

Summary

Adds reusable React error boundaries around the dashboard and major layout sections so render-time failures in one section do not take down the whole UI.

Changes

• Added ErrorBoundary as a reusable class component using getDerivedStateFromError and componentDidCatch.
• Added clean fallback UI styles that match the existing dark design tokens.
• Wrapped Header, Sidebar, and main route content in separate layout-level boundaries.
• Wrapped the dashboard route in its own boundary.
• Split dashboard stat rendering into a StatCard component and wrapped the stat grid in a distinct boundary.

Testing

• npm install
• npm run build
• PYTHONUTF8=1 python -u build.py -m frontend --verbose

Validation passed for the affected frontend module.

Diagnostic artifacts generated by build.py for commit 0dde0a5:

• diagnostic/build-0dde0a52.json
• diagnostic/build-0dde0a52-part001.logd
• diagnostic/build-0dde0a52-part002.logd
• diagnostic/build-0dde0a52-part003.logd
• diagnostic/build-0dde0a52-part004.logd
• diagnostic/build-0dde0a52-part005.logd
• diagnostic/build-0dde0a52-part006.logd

Diagnostic password:

994378c16a1a357d6bdf

Note: on this Windows environment, Python subprocess could not launch npm unless the build command used npm.cmd. I used that as a local validation workaround and did not include it in the frontend patch.

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic build log
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

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• I would like to request that my diagnostic build log is removed before merging