fix(reports): add scoped error boundary

[Kandexa]

Summary

Closes #1166
Closes #1168
Closes #1169
Closes #1167

Adds the missing reports-level error boundary with a reports-specific retry action and dashboard navigation. This keeps a failed reports fetch within the reports segment instead of using the broader protected-layout fallback.

Why

The reports route had a loading boundary but no matching error boundary, so a reports-only data failure could surface through a generic protected-route error. A nested error.tsx uses the existing App Router boundary pattern and gives users a retry path local to the failed segment.

What was built

File	What it contains
frontend/app/[locale]/(protected)/reports/error.tsx	Client error boundary that logs the failure, invokes Next.js reset() for retry, and links back to the dashboard.
frontend/test-utils/reports-error.test.tsx	Verifies reports-scoped messaging, retry behavior, and dashboard navigation.

Integration changes outside frontend/app/[locale]/(protected)/reports/

No existing production files modified; the route boundary and its test are additions only.

Acceptance criteria coverage

• A failed reports data fetch shows a reports-scoped error state with a retry action (frontend/app/[locale]/(protected)/reports/error.tsx plus frontend/test-utils/reports-error.test.tsx — scoped heading and retry callback)

Deliberately deferred

None.

Test plan

• npm test — not run per request
• npm run typecheck — not run per request
• npm run lint — not run per request
• npm run build — not run per request
• Formatter — not run per request

Env vars / Notes

No new environment variables or public API surfaces. The parent protected error boundary remains unchanged for failures outside the reports segment.

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[mftee]

No merge conflicts with main. Scoped error boundary for reports is a solid fix - approving.