feat: add offline support with data caching, connectivity monitoring,… - #1138

Description

Adds an offline-aware read mode for previously loaded contracts on the contract detail page (/contracts/[id]). Temporary connectivity loss no longer erases loaded contract data or leaves the page in an unusable blank state.

Key capabilities introduced:

• Bounded & Versioned Storage Snapshot: Caches contract read models in localStorage up to a bounded 50 entries (MAX_CACHE_ENTRIES = 50) using chronological LRU eviction, schemaVersion: 1, and staleness detection (5-minute threshold).
• Corruption Resilience & SSR Safety: Detects and recovers from invalid schema or corrupt JSON in storage without crashing; guards all browser API calls with isBrowser().
• Dynamic Online/Offline Detection: useOnlineStatus hook tracks browser network changes in real time via window online/offline events.
• Accessible Warning Banner: OfflineIndicator displays offline and stale state notices with relative cached timestamps ("Cached 5 minutes ago") with role="status" and aria-live="polite".
• Mutation Disabling: Strictly disables unsafe write actions (Submit Milestone, Release Funds, Dispute, inline milestone edits) when offline or viewing stale snapshots, with clear tooltips and error toasts.
• Automatic Re-sync: Seamlessly fetches fresh contract data from the upstream resolver/repository and updates the cache when the network connection returns.

Closes #1131

Type of Change

• Bug fix (non-breaking change that resolves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (no functional change, internal code improvement)
• Documentation update

Pre-flight Checklist

All items must be checked before requesting review.

• npm run lint passes with no errors
• npm test passes with no failures
• npm run build completes successfully

Testing & Coverage

• Module test coverage for impacted files meets or exceeds the 95% minimum threshold
  (run npm test -- --coverage and check the per-file table)
• If this PR adds or modifies UI components, accessibility tests were written using
  the shared utilities in src/test-utils/a11y.tsx

What was tested?

• contractCache.test.ts (14 unit tests):
  • Cache snapshot saving, retrieval, and updating.
  • Staleness evaluation before and after 5-minute threshold.
  • Bounded storage LRU eviction when exceeding 50 entries.
  • Corruption resilience (non-JSON string, malformed schema, corrupted entries).
  • Quota exceeded and storage removal error handling.
• useOnlineStatus.test.ts (6 unit tests):
  • Initialization from navigator.onLine (including undefined fallback).
  • Real-time reactivity on window offline and online events.
  • Clean event listener teardown on unmount.
• OfflineIndicator.test.tsx (9 component & a11y tests):
  • Hidden when online with fresh data; rendered when offline or stale.
  • Relative timestamp formatting ("just now", "10 minutes ago", "2 hours ago", "3 days ago").
  • jest-axe automated accessibility audits with 0 violations.
• ActionPanel.test.tsx (41 unit & a11y tests):
  • Verified disableMutations prop disables submit, release, and dispute actions with informative tooltip titles while keeping View Summary accessible.
  • assertNoA11yViolations checks in normal, open dispute form, and mutation-disabled states.
• offlineReadMode.test.tsx & page.test.tsx (52 integration tests):
  • Offline on first load: Uncached contract displays clean offline error state and disables mutations.
  • Offline after data loaded: Cached contract displays snapshot with offline indicator and disables unsafe mutations and milestone edits.
  • Stale snapshot: Stale data banner displayed, milestone edits and status changes blocked.
  • Cache corrupt: Malformed localStorage parsed safely without unhandled errors.
  • Connection returns: online event triggers automatic fresh data re-fetch, clears offline banner, and re-enables mutations.
• Full Suite: All 150 test suites passed (3,076 tests).

Accessibility & Security Notes

Accessibility

• Ran automated jest-axe accessibility scans and assertNoA11yViolations against OfflineIndicator and ActionPanel in all states (0 violations).
• OfflineIndicator uses role="status", aria-live="polite", and aria-label to announce network state changes cleanly without disrupting screen reader focus.
• Action buttons have descriptive title and aria-describedby tooltips explaining why mutations are disabled when offline or viewing stale snapshots.

Security

• Boundaries & Isolation: Cache only stores public contract read models (ContractData). Wallet private keys, secrets, session authentication tokens, and tenant boundaries are never stored in localStorage.
• No Offline Writes: Write operations are blocked at the component and handler level when offline or stale to prevent data inconsistency or replay conflicts.
• Sanitized Presentation: User-supplied strings continue to use safe boundaries and standard React DOM escaping.