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>

src/contexts/AnonymousContext.tsx

@@ -0,0 +1,34 @@
+"use client";
+
+import React, { createContext, useContext, useMemo, useCallback } from "react";
+import { usePreferences } from "@/contexts/PreferencesContext";
+
+interface AnonymousContextType {
+  isAnonymous: boolean;
+  toggleAnonymous: () => void;
+}
+
+const AnonymousContext = createContext<AnonymousContextType | undefined>(undefined);
+
+export function AnonymousProvider({ children }: { children: React.ReactNode }) {
+  const { preferences, updatePreferences } = usePreferences();
+
+  const toggleAnonymous = useCallback(() => {
+    updatePreferences({ anonymousMode: !preferences.anonymousMode });
+  }, [preferences.anonymousMode, updatePreferences]);
+
+  const contextValue = useMemo(
+    () => ({ isAnonymous: preferences.anonymousMode, toggleAnonymous }),
+    [preferences.anonymousMode, toggleAnonymous]
+  );
+
+  return <AnonymousContext.Provider value={contextValue}>{children}</AnonymousContext.Provider>;
+}
+
+export function useAnonymous() {
+  const context = useContext(AnonymousContext);
+  if (context === undefined) {
+    throw new Error("useAnonymous must be used within an AnonymousProvider");
+  }
+  return context;
+}