feat: add TypeScript types for highlights and sync system

Added highlight system types with strict color and sync status validation:
- HighlightColor type with 4 valid colors (yellow, orange, pink, blue)
- SyncStatus type for tracking sync state (pending, syncing, synced, error)
- BibleHighlight interface with full metadata support
- HighlightSyncQueueItem for offline sync queue management
- CrossReference interface for verse cross-referencing

Includes comprehensive test coverage validating type constraints.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

types/index.ts

@@ -94,3 +94,33 @@ export interface CacheEntry {
   timestamp: number
   expiresAt: number
 }
+
+// Highlight system types
+export type HighlightColor = 'yellow' | 'orange' | 'pink' | 'blue'
+export type SyncStatus = 'pending' | 'syncing' | 'synced' | 'error'
+
+export interface BibleHighlight {
+  id: string // UUID
+  verseId: string
+  userId?: string // Optional, added by backend
+  color: HighlightColor
+  createdAt: number // timestamp
+  updatedAt: number // timestamp
+  syncStatus: SyncStatus
+  syncErrorMsg?: string
+}
+
+export interface HighlightSyncQueueItem {
+  highlightId: string
+  action: 'create' | 'update' | 'delete'
+  highlight: BibleHighlight
+  retryCount: number
+}
+
+export interface CrossReference {
+  refVerseId: string
+  bookName: string
+  chapter: number
+  verse: number
+  preview: string
+}