fix: add Tailwind v4 import directive to index.css

Critical bug fix (P0): Tailwind CSS classes were not being compiled.

Root Cause:
- Tailwind v4 requires explicit @import tailwindcss in CSS file
- File was missing this directive, causing all utility classes to be ignored
- CSS bundle was only 4KB instead of expected 30KB+

Fix:
- Added @import tailwindcss; at the top of src/index.css
- Verified build now generates 30KB+ CSS bundle with all utility classes
- Confirmed classes like text-3xl, bg-indigo-600, max-w-7xl are present

Safety First:
- Build passes successfully
- No breaking changes to existing styles
- Incremental fix before UI refactoring

Refs: Tailwind v4 migration guide

frontend/src/index.css

@@ -1,3 +1,5 @@
+@import "tailwindcss";
+
 :root {
   --text: #6b6375;
   --text-h: #08060d;