feat(ux): Sprint 3 mobile nav, sync conflicts, and resilience

Improve mobile bottom navigation, accessible dialogs and cards, explicit
sync conflict resolution, i18n error messages, encrypted draft autosave,
and persistent storage hints for offline data safety.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/services/syncConflicts.ts

@@ -0,0 +1,48 @@
+export interface SyncConflict {
+  logbookId: string
+  payloadId: string
+  type: string
+  reason: string
+  queueItemId?: number
+  detectedAt: string
+}
+
+const conflicts = new Map<string, SyncConflict>()
+const listeners = new Set<() => void>()
+
+function conflictKey(logbookId: string, payloadId: string, type: string): string {
+  return `${logbookId}:${type}:${payloadId}`
+}
+
+export function getSyncConflicts(logbookId?: string): SyncConflict[] {
+  const all = Array.from(conflicts.values())
+  if (!logbookId) return all
+  return all.filter((c) => c.logbookId === logbookId)
+}
+
+export function hasSyncConflicts(logbookId?: string): boolean {
+  return getSyncConflicts(logbookId).length > 0
+}
+
+export function reportSyncConflict(conflict: Omit<SyncConflict, 'detectedAt'>): void {
+  const key = conflictKey(conflict.logbookId, conflict.payloadId, conflict.type)
+  conflicts.set(key, { ...conflict, detectedAt: new Date().toISOString() })
+  listeners.forEach((l) => l())
+}
+
+export function clearSyncConflict(logbookId: string, payloadId: string, type: string): void {
+  conflicts.delete(conflictKey(logbookId, payloadId, type))
+  listeners.forEach((l) => l())
+}
+
+export function clearSyncConflictsForLogbook(logbookId: string): void {
+  for (const key of conflicts.keys()) {
+    if (key.startsWith(`${logbookId}:`)) conflicts.delete(key)
+  }
+  listeners.forEach((l) => l())
+}
+
+export function subscribeSyncConflicts(listener: () => void): () => void {
+  listeners.add(listener)
+  return () => listeners.delete(listener)
+}