fix(frontend): add dashboard section fallbacks

[mralonsocorona]

Summary

Implements robust dashboard section fallbacks for the React frontend.

Changes included:

• Added a reusable SectionFallback component for render-time section failures.
• Wrapped Header, Sidebar, and main app content in Layout.
• Wrapped Dashboard stats and Dashboard panels independently so one broken section does not crash the whole layout.
• Added scoped fallback styles matching the existing dashboard/app layout naming.
• Included generated diagnostics for the validated frontend build.

Validation

Validated locally and through the repository diagnostic flow:

• python3 -m py_compile build.py
• python3 build.py --help
• cd frontend && npm run build
• python3 build.py -m frontend inside Ubuntu 24.04 Docker container

Diagnostic artifacts included:

• diagnostic/build-351208e6.json
• diagnostic/build-351208e6.logd

Diagnostic decrypt command:

encryptly unpack diagnostic/build-351208e6.logd <outdir> --password 07c5642fac692758929f

Notes

• No package files, generated tsbuildinfo, dist, or dependency folders are included.
• No backend, workflow, binary, or build tool changes were made.
• The implementation uses project-context naming (SectionFallback) and scoped layout/dashboard fallback classes.