Add saved filter menu items for custom gallery sections

[Alb3e3]

Summary

• Adds configurable main navigation entries backed by saved gallery filters.
• Routes custom entries to the existing gallery list using Stash's saved-filter URL serialization.
• Adds Interface settings controls to add or remove saved gallery filters as menu items, so users can create sections such as Manga or Doujin without adding a new content type.

Notes

This keeps the implementation aligned with the existing gallery/filter model instead of introducing a separate Manga/Doujin page or schema.

Addresses #1659.

Verification

• Focused helper assertion via esbuild + node
• npm run check
• npm run lint:js
• npm run lint:css
• npm run format-check
• npm run build (passes with existing Sass deprecation warnings)
• git diff --cached --check