feat(webui): Enhance dashboard, global styles, and settings module

## Dashboard Enhancements
- Add Request Volume trend chart with Chart.js line graph
  - Support Family/Model display modes for aggregation levels
  - Show Total/Today/1H usage statistics
  - Hierarchical filter dropdown with Smart select (Top 5 by 24h usage)
  - Persist chart preferences to localStorage
- Improve account health detection logic
  - Core models (sonnet/opus/pro/flash) require >5% quota to be healthy
  - Dynamic quota ring chart supporting any model family
- Unify table styles with standard-table class

## Global Style Refactoring
- Add CSS variable system for theming
  - Space color scale (950/900/850/800/border)
  - Neon accent colors (purple/green/cyan/yellow/red)
  - Text hierarchy (main/dim/muted/bright)
  - Chart palette (16 colors)
- Add unified component classes
  - .view-container for consistent page layouts
  - .section-header/.section-title/.section-desc
  - .standard-table for table styling
- Update scrollbar, nav-item, progress-bar to use theme variables

## Settings Module Extensions
- Add model mapping column in Models tab
- Enhance model selectors with family color indicators
- Support horizontal scroll for tabs on narrow screens
- Add defaultCooldownMs and maxWaitBeforeErrorMs config options

## New Module
- Add src/modules/usage-stats.js for request tracking
  - Track /v1/messages and /v1/chat/completions endpoints
  - Hierarchical storage: { hour: { family: { model: count } } }
  - Auto-save every minute, 30-day retention
  - GET /api/stats/history endpoint for dashboard chart

## Backend Changes
- Add direct account manipulation helpers (bypass AccountManager)
- Add POST /api/config/password endpoint for WebUI password change
- Auto-reload AccountManager after account operations
- Use CSS variables in OAuth callback pages

## Other
- Update .gitignore for runtime data directory
- Add i18n keys for new UI elements (EN/zh_CN)

Co-Authored-By: Claude <noreply@anthropic.com>

package-lock.json

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