Add JSDoc comments to store files

[beihaili]

Task

Add JSDoc comments to all Zustand store files to improve developer experience and code readability.

What to do

• Add JSDoc @typedef for each store's state shape
• Document each action/method with @param and @returns
• Add a module-level @module description for each store file
• Explain what each store is responsible for

Files to look at

• src/store/useContentStore.js — content fetching and caching
• src/store/useUserStore.js — user progress, badges, streaks
• src/store/useAppStore.js — app settings, API key
• src/store/useSearchStore.js — search history and state

What a good PR looks like

• Every exported function and state property is documented
• JSDoc types are accurate and match actual usage
• Comments are concise but helpful
• No functional changes to the code