refactor: migrate to Tailwind CSS 4 native configuration

Convert from Tailwind CSS 3 style to Tailwind CSS 4 native approach:
- Install @tailwindcss/postcss plugin
- Create postcss.config.mjs with Tailwind PostCSS plugin
- Migrate all custom theme colors to globals.css using @theme directive
- Remove tailwind.config.ts (no longer needed in Tailwind CSS 4)
- Update globals.css from @import style to native CSS with custom properties

Custom theme includes:
- 23 category-specific colors as CSS custom properties
- --color-category-* naming convention for all unit types
- Follows Tailwind CSS 4 best practices with @theme block

Benefits:
- Native CSS features, no JS config needed
- Better performance with PostCSS plugin
- Cleaner separation of concerns
- Follows Tailwind CSS 4 migration guide

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tsconfig.json

@@ -1,7 +1,11 @@
 {
   "compilerOptions": {
     "target": "ES2020",
-    "lib": ["DOM", "DOM.Iterable", "ES2020"],
+    "lib": [
+      "DOM",
+      "DOM.Iterable",
+      "ES2020"
+    ],
     "allowJs": true,
     "skipLibCheck": true,
     "strict": true,
@@ -11,7 +15,7 @@
     "moduleResolution": "bundler",
     "resolveJsonModule": true,
     "isolatedModules": true,
-    "jsx": "preserve",
+    "jsx": "react-jsx",
     "incremental": true,
     "plugins": [
       {
@@ -19,9 +23,19 @@
       }
     ],
     "paths": {
-      "@/*": ["./*"]
+      "@/*": [
+        "./*"
+      ]
     }
   },
-  "include": ["next-env.d.ts", "**/*.ts", "**/*.tsx", ".next/types/**/*.ts"],
-  "exclude": ["node_modules"]
+  "include": [
+    "next-env.d.ts",
+    "**/*.ts",
+    "**/*.tsx",
+    ".next/types/**/*.ts",
+    ".next/dev/types/**/*.ts"
+  ],
+  "exclude": [
+    "node_modules"
+  ]
 }