feat: add anonymous mode toggle to hide reading progress and tracking

Adds a toggleable anonymous mode (eye icon in header) that:
- Stops syncing read progress to the server while reading
- Hides mark as read/unread buttons on book covers and lists
- Hides reading status badges on series and books
- Hides progress bars on series and book covers
- Hides "continue reading" and "continue series" sections on home
- Persists the setting server-side in user preferences (anonymousMode)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -64,6 +64,7 @@ model Preferences {
   displayMode         Json
   background          Json
   readerPrefetchCount Int      @default(5)
+  anonymousMode       Boolean  @default(false)
   createdAt           DateTime @default(now())
   updatedAt           DateTime @updatedAt