Persist language filter preference in localStorage

[realproject7]

Summary

When a user sets a language filter (e.g. Korean), it resets to the default on revisit. The filter preference should persist across sessions using localStorage.

Implementation

• Save the selected language filter to localStorage (e.g. key: plotlink-lang-filter)
• On page load, read from localStorage and use as the initial filter value
• If no saved preference, default to "all"
• Apply the same pattern to writer filter and genre filter if they exist as persistent preferences

Acceptance Criteria

• Language filter persists across page reloads and revisits
• Stored in localStorage (not cookies)
• Default is "all" when no saved preference exists
• Changing the filter updates localStorage immediately

[realproject7]

Fixed via PR #557. Language filter now persists in localStorage across sessions.