fix(routes): unify not-found boundaries

[kilodesodiq-arch]

Summary

Closes #1173
Closes #1170
Closes #1171
Closes #1172

Unifies the root, locale, and document-specific not-found boundaries around one shared branded presentation while preserving translated copy in the locale boundary. Boundaries without locale context use the default English copy instead of maintaining a second inconsistent visual implementation.

Why

The three not-found boundaries duplicated markup and diverged in layout, link styling, and messaging. Centralizing the structure removes that drift while keeping the locale boundary's existing next-intl message source and the document boundary's distinct missing-anchor explanation.

What was built

File	What it contains
frontend/components/NotFoundContent.tsx	Shared 404 layout with stable branding, home navigation, and optional detail copy.
frontend/app/not-found.tsx	Root boundary using the default English copy through the shared presentation.
frontend/app/[locale]/not-found.tsx	Locale boundary using the existing translated notFound messages.
frontend/app/(protected)/documents/[id]/not-found.tsx	Document-specific boundary using consistent branding and default-locale missing-anchor copy.
frontend/test-utils/not-found.test.tsx	Render coverage for all three boundaries and their copy/navigation contracts.

Integration changes outside frontend/components/

• frontend/app/not-found.tsx — replaced duplicated root markup with the shared presentation.
• frontend/app/[locale]/not-found.tsx — retained locale-aware message lookup while using the shared presentation.
• frontend/app/(protected)/documents/[id]/not-found.tsx — replaced the divergent document-specific markup with the shared presentation.

Acceptance criteria coverage

• All three not-found boundaries render consistent branding and appropriately localized copy (frontend/components/NotFoundContent.tsx, all three boundary files, and frontend/test-utils/not-found.test.tsx — root/default, locale/translated, and document-specific render coverage)

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 root and document boundaries intentionally use default English copy because those boundaries do not receive a locale parameter; the locale boundary continues to resolve notFound translations through next-intl.

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[mftee]

No merge conflicts with main. Unifying not-found boundaries across routes is a good consistency fix - approving.