fix: resolve Tailwind CSS v4 import on Windows

Use `@import "tailwindcss/index.css"` instead of `@import "tailwindcss"`
for cross-platform compatibility. Remove unnecessary `@source "./src"`
directive and stale `tailwind.config.js` (Tailwind v4 auto-detects content).

Closes #11821

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/tailwind.css

@@ -1,3 +1 @@
-@import "tailwindcss";
-
-@source "./src";
+@import "tailwindcss/index.css";

tailwind.config.js

@@ -1,3 +0,0 @@
-module.exports = {
-  content: ['./src/**/*.tsx'],
-};