feat(settings): add left navigation sidebar to settings modal

Implement two-column layout for improved settings navigation:
- Add 200px fixed navigation sidebar with 4 groups (General, Interface, Download, Advanced)
- Implement scroll spy to highlight current section during scroll
- Add smooth scrolling when clicking navigation items
- Extend modal width from 700px to 950px for better content display
- Add responsive mobile layout (switches to stacked view below 768px)
- Add i18n keys for navigation group titles
- Create documentation for optimization phases and progress tracking

Files changed:
- settings-modal.css: Add sidebar, navigation, and responsive styles
- settings_modal.html: Restructure with two-column layout and section IDs
- SettingsManager.js: Add initializeNavigation() with scroll spy
- locales/*.json: Add settings.nav translations (en, zh-CN, zh-TW, ja, ru, de, fr, es, ko, he)
- docs/ui-ux-optimization/: Add proposal and progress tracker documentation

locales/ja.json

@@ -269,6 +269,12 @@
             "storageLocation": "設定の場所",
             "proxySettings": "プロキシ設定"
         },
+        "nav": {
+            "general": "[TODO: Translate] General",
+            "interface": "[TODO: Translate] Interface",
+            "download": "[TODO: Translate] Download",
+            "advanced": "[TODO: Translate] Advanced"
+        },
         "storage": {
             "locationLabel": "ポータブルモード",
             "locationHelp": "有効にすると settings.json をリポジトリ内に保持し、無効にするとユーザー設定ディレクトリに格納します。"