Add offline assessments plan and architecture documentation (#451)

* Add offline assessments implementation plan

Four-step plan for offline inspection/assessment support:
1. Service worker + page caching
2. Offline state detection + UI (badge ID field)
3. Offline form queue (IndexedDB)
4. Background sync + badge ID resolution

https://claude.ai/code/session_01PgJyYovGyfvzyghBFMDjA2

* Add browser compatibility section to offline plan

Documents Background Sync API as Chromium-only, Safari cache eviction
behaviour, and the recommendation to use the online event listener as
the primary sync mechanism with Background Sync as progressive
enhancement.

https://claude.ai/code/session_01PgJyYovGyfvzyghBFMDjA2

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Co-authored-by: Claude <noreply@anthropic.com>

Author: stefan-burke