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

docs/ui-ux-optimization/progress-tracker.md

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+# Settings Modal Optimization Progress Tracker
+
+## Project Overview
+**Goal**: Optimize Settings Modal UI/UX with left navigation sidebar
+**Started**: 2026-02-23
+**Current Phase**: P0 - Left Navigation Sidebar
+
+---
+
+## Phase 0: Left Navigation Sidebar (P0)
+
+### Status: Completed ✓
+
+### Completion Notes
+- All CSS changes implemented
+- HTML structure restructured successfully  
+- JavaScript navigation functionality added
+- Translation keys added and synchronized
+- Ready for testing and review
+
+### Tasks
+
+#### 1. CSS Changes
+- [x] Add two-column layout styles
+  - [x] `.settings-modal` flex layout
+  - [x] `.settings-nav` sidebar styles
+  - [x] `.settings-content` content area styles
+  - [x] `.settings-nav-item` navigation item styles
+  - [x] `.settings-nav-item.active` active state styles
+- [x] Adjust modal width to 950px
+- [x] Add smooth scroll behavior
+- [x] Add responsive styles for mobile
+- [x] Ensure dark theme compatibility
+
+#### 2. HTML Changes
+- [x] Restructure modal HTML
+  - [x] Wrap content in two-column container
+  - [x] Add navigation sidebar structure
+  - [x] Add navigation items for each section
+  - [x] Add ID anchors to each section
+- [x] Update section grouping if needed
+
+#### 3. JavaScript Changes
+- [x] Add navigation click handlers
+- [x] Implement smooth scroll to section
+- [x] Add scroll spy for active nav highlighting
+- [x] Handle nav item click events
+- [x] Update SettingsManager initialization
+
+#### 4. Translation Keys
+- [x] Add translation keys for navigation groups
+  - [x] `settings.nav.general`
+  - [x] `settings.nav.interface`
+  - [x] `settings.nav.download`
+  - [x] `settings.nav.advanced`
+
+#### 4. Testing
+- [x] Verify navigation clicks work
+- [x] Verify active highlighting works
+- [x] Verify smooth scrolling works
+- [ ] Test on mobile viewport (deferred to final QA)
+- [ ] Test dark/light theme (deferred to final QA)
+- [x] Verify all existing settings work
+- [x] Verify save/load functionality
+
+### Blockers
+None currently
+
+### Notes
+- Started implementation on 2026-02-23
+- Following existing design system and CSS variables
+
+---
+
+## Phase 1: Section Collapse/Expand (P1)
+
+### Status: Planned
+
+### Tasks
+- [ ] Add collapse/expand toggle to section headers
+- [ ] Add chevron icon with rotation animation
+- [ ] Implement localStorage for state persistence
+- [ ] Add CSS animations for smooth transitions
+
+---
+
+## Phase 2: Search Bar (P1)
+
+### Status: Planned
+
+### Tasks
+- [ ] Add search input to header area
+- [ ] Implement real-time filtering
+- [ ] Add highlight for matched terms
+- [ ] Handle empty search results
+
+---
+
+## Phase 3: Visual Hierarchy (P2)
+
+### Status: Planned
+
+### Tasks
+- [ ] Add accent border to section headers
+- [ ] Bold setting labels
+- [ ] Increase section spacing
+
+---
+
+## Phase 4: Quick Actions (P3)
+
+### Status: Planned
+
+### Tasks
+- [ ] Add reset to defaults button
+- [ ] Add export config button
+- [ ] Add import config button
+- [ ] Implement corresponding functionality
+
+---
+
+## Change Log
+
+### 2026-02-23
+- Created project documentation
+- Started Phase 0 implementation
+- Analyzed existing code structure
+- Implemented two-column layout with left navigation sidebar
+- Added CSS styles for navigation and responsive design
+- Restructured HTML to support new layout
+- Added JavaScript navigation functionality with scroll spy
+- Added translation keys for navigation groups
+- Synchronized translations across all language files
+- Tested in browser - navigation working correctly
+
+---
+
+## Testing Checklist
+
+### Functional Testing
+- [ ] All settings save correctly
+- [ ] All settings load correctly
+- [ ] Navigation scrolls to correct section
+- [ ] Active nav updates on scroll
+- [ ] Mobile responsive layout
+
+### Visual Testing
+- [ ] Design matches existing UI
+- [ ] Dark theme looks correct
+- [ ] Light theme looks correct
+- [ ] Animations are smooth
+- [ ] No layout shifts or jumps
+
+### Cross-browser Testing
+- [ ] Chrome/Chromium
+- [ ] Firefox
+- [ ] Safari (if available)