refactor(styles): migrate from CSS variables to Tailwind classes

- Replace all CSS custom properties with Tailwind utility classes
- Add tailwind.config.js with custom color palette matching Linear design
- Reduce index.css from 89 to 37 lines (58% reduction)
- Maintain consistent visual appearance with semantic color usage
- Update all components to use Tailwind classes instead of CSS variables

tailwind.config.js

@@ -0,0 +1,58 @@
+/** @type {import('tailwindcss').Config} */
+export default {
+  content: [
+    "./index.html",
+    "./src/**/*.{js,ts,jsx,tsx}",
+  ],
+  theme: {
+    extend: {
+      colors: {
+        // 扩展蓝色系列以匹配 Linear 风格
+        blue: {
+          500: '#3498db',
+          600: '#2980b9',
+          400: '#5dade2',
+        },
+        // 自定义灰色系列
+        gray: {
+          50: '#fafafa',   // bg-primary
+          100: '#f4f4f5',  // bg-tertiary
+          200: '#e4e4e7',  // border
+          300: '#d4d4d8',  // border-hover
+          400: '#a1a1aa',  // text-tertiary
+          500: '#71717a',  // text-secondary
+          900: '#18181b',  // text-primary
+        },
+        // 状态颜色
+        green: {
+          500: '#10b981',
+          100: '#d1fae5',
+        },
+        red: {
+          500: '#ef4444',
+          100: '#fee2e2',
+        },
+        amber: {
+          500: '#f59e0b',
+          100: '#fef3c7',
+        },
+      },
+      boxShadow: {
+        'sm': '0 1px 2px 0 rgb(0 0 0 / 0.05)',
+        'md': '0 4px 6px -1px rgb(0 0 0 / 0.1), 0 2px 4px -2px rgb(0 0 0 / 0.1)',
+        'lg': '0 10px 15px -3px rgb(0 0 0 / 0.1), 0 4px 6px -4px rgb(0 0 0 / 0.1)',
+      },
+      borderRadius: {
+        'sm': '0.375rem',
+        'md': '0.5rem',
+        'lg': '0.75rem',
+        'xl': '0.875rem',
+      },
+      fontFamily: {
+        sans: ['-apple-system', 'BlinkMacSystemFont', '"Segoe UI"', 'Roboto', '"Helvetica Neue"', 'Arial', 'sans-serif'],
+        mono: ['ui-monospace', 'SFMono-Regular', '"SF Mono"', 'Consolas', '"Liberation Mono"', 'Menlo', 'monospace'],
+      },
+    },
+  },
+  plugins: [],
+}