chore: initial baseline with P0-safety .gitignore

docs/diagrams/sequence-diagram.mermaid

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+sequenceDiagram
+    participant U as 坐席
+    participant TB as TopBar.vue
+    participant TS as useThemeStore
+    participant UT as useTheme.ts
+    participant DOM as document.documentElement
+    participant LS as localStorage
+
+    U->>TB: 点击 ☀️/🌙 切换开关
+    TB->>TS: toggleTheme()
+    TS->>TS: currentTheme = currentTheme === 'light' ? 'dark' : 'light'
+    TS->>UT: applyTheme(currentTheme)
+    UT->>DOM: setAttribute('data-theme', theme)
+    UT->>LS: setItem('theme', theme)
+    DOM-->>DOM: CSS 变量自动切换（:root / [data-theme="dark"]）
+    DOM-->>U: 300ms 过渡动画，界面变色
+
+    Note over U,LS: 页面加载时
+    U->>UT: 首次进入页面
+    UT->>LS: getItem('theme')
+    LS-->>UT: 'dark' | 'light' | null
+    UT->>DOM: setAttribute('data-theme', theme || 'light')