feat(theme): 专业主题系统重构 — Warp/Linear 设计参考，深色/浅色完整适配

参考设计系统:
- Warp: 温暖暗色调、自然层次、克制配色
- Linear: 亮度阶梯层次、半透明边框、精确排版

核心改进:
- app-theme.ts: 完整重写深色/浅色调色板 (+surface-hover/active, border-subtle/focus, feedback-bg, placeholder)
- useTheme.ts: 修复硬编码 return 'dark'，支持 dark/light/system 完整切换
- index.css: 消除硬编码颜色，全部使用 CSS 变量
- tailwind.config.js: 新增 txt/content/surface/border/ph/feedback 语义 token
- 44 文件批量迁移: 292 处 text-gray/bg-gray/text-white/placeholder-gray → 主题感知语义 token

| 44 files +611/-482 | tsc零错误 | 96/96 tests passing

src/hooks/useTheme.ts

@@ -6,9 +6,12 @@ import { darkColors, lightColors, type AppThemeMode, type ColorMap } from '../th
  * Resolves the *effective* theme ('dark' | 'light') from the user preference,
  * including the 'system' → prefers-color-scheme mapping.
  */
-function resolveEffectiveTheme(_preference: AppThemeMode): 'dark' | 'light' {
-  // 当前版本固定使用 dark 主题，后续版本开放主题切换
-  return 'dark'
+function resolveEffectiveTheme(preference: AppThemeMode): 'dark' | 'light' {
+  if (preference === 'system') {
+    if (typeof window === 'undefined') return 'dark'
+    return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
+  }
+  return preference
 }
 
 /**
@@ -38,8 +41,10 @@ export function useTheme() {
   useEffect(() => {
     const colors = effective === 'dark' ? darkColors : lightColors
 
-    // Set data attribute for potential direct CSS selectors
+    // Set data attribute for CSS selectors
     document.documentElement.setAttribute('data-theme', effective)
+
+    // Toggle Tailwind dark class
     if (effective === 'dark') {
       document.documentElement.classList.add('dark')
     } else {
@@ -49,9 +54,9 @@ export function useTheme() {
     // Apply CSS custom properties
     applyCSSVars(colors)
 
-    // Update body bg & colour (mirrors index.css body rules but via vars)
-    document.body.style.backgroundColor = '#0f172a'
-    document.body.style.color = '#e2e8f0'
+    // Update body bg & colour from tokens (not hardcoded)
+    document.body.style.backgroundColor = colors['surface-base'] || '#0f172a'
+    document.body.style.color = colors['text-primary'] || '#e2e8f0'
   }, [effective])
 
   // Listen for system colour-scheme changes when preference is 'system'
@@ -64,9 +69,16 @@ export function useTheme() {
       const newEffective = e.matches ? 'dark' : 'light'
       const colors = e.matches ? darkColors : lightColors
       document.documentElement.setAttribute('data-theme', newEffective)
+
+      if (newEffective === 'dark') {
+        document.documentElement.classList.add('dark')
+      } else {
+        document.documentElement.classList.remove('dark')
+      }
+
       applyCSSVars(colors)
-      document.body.style.backgroundColor = '#0f172a'
-      document.body.style.color = '#e2e8f0'
+      document.body.style.backgroundColor = colors['surface-base'] || '#0f172a'
+      document.body.style.color = colors['text-primary'] || '#e2e8f0'
     }
 
     mql.addEventListener('change', handler)