refactor(settings): restructure settings modal with subsection headers

- Replace duplicate section headers with meaningful subsection titles
- Group settings under logical subsections using existing i18n keys
- Add new translation key 'settings.sections.apiConfiguration'
- Update CSS for subsection styling with proper visual hierarchy
- Improve UX by making settings organization clearer

Subsections now use familiar titles from existing translations:
- API Configuration, Storage Location, Language (General)
- Content Filtering, Video Settings, Layout Settings (Interface)
- Folder Settings, Download Path Templates, Priority Tags,
  Update Flags, Example Images (Download)
- Auto-organize Exclusions, Metadata Refresh Skip Paths (Organization)
- Metadata Archive, Misc (System)
- Proxy Settings (Network)

locales/ko.json

@@ -255,6 +255,7 @@
             "clipboardFallback": "설정 경로: {{path}}"
         },
         "sections": {
+            "apiConfiguration": "[TODO: Translate] API Configuration",
             "contentFiltering": "콘텐츠 필터링",
             "videoSettings": "비디오 설정",
             "layoutSettings": "레이아웃 설정",