feat(offline): deterministic conflict resolution and idempotent sync convergence

[zakariyaufarida5-wq]

Summary

Makes offline sync deterministic and convergent for the IndexedDB queue (closes #1144):

• Deterministic conflict resolution — per-record versioning with a version vector (version + updatedBy + Lamport logical clock) replaces pure timestamp LWW. Merges are stable regardless of device clock drift, with per-entity-type merge strategies (course_progress uses max-progress / OR-completion).
• Idempotent replay — every queued operation carries a client-generated operationId; acknowledged ids are persisted in a dedupe map so duplicate delivery applies exactly once. Retries use capped exponential backoff, and operations that exhaust their retry cap move to a dead-letter queue (with manual retry).
• Transactional batch drain — batches drain all-or-nothing inside a single IndexedDB transaction. A partial failure rolls back cleanly (no queue/ack/progress writes), and a persisted resumable cursor survives app restarts.
• UI-facing conflict state — useOfflineSync / useOfflineMode expose pending / conflicted / resolved counts plus conflicts; the ConflictResolver displays the state and both version vectors. Acked records are garbage-collected per the retention policy, and the service worker triggers a drain via background sync when connectivity returns.
• Store reconciliation — after each drain the persisted store (synchronizationEngine / persistenceLayer) is updated with lastSynced and syncStatus and broadcast across tabs.

Tests

• src/services/__tests__/offlineSync.test.ts — idempotent replay under duplicate delivery, operationId propagation, batch rollback on partial failure, cursor resume after restart, dead-lettering + retry, in-drain backoff retry, deterministic auto-merge across clock skew, manual conflict surfacing/resolution, retention GC, sync status.
• src/lib/conflict/__tests__/resolver.test.ts — version-vector comparison, deterministic/commutative merges across reordered and clock-skewed inputs, per-entity strategies, legacy fallback.
• Existing src/testing/conflict.test.ts and OfflineSyncManager.test.ts continue to pass (the latter was previously unloadable due to a broken relative import, now fixed).

CI

Type-check, lint, validate:ui, validate:web3, and next build all pass locally.

closes #1144