chore: run Tailwind v4 upgrade tool (Phase 1)

- Upgraded tailwindcss v3 → v4.2.1, postcss plugin to @tailwindcss/postcss
- Deleted tailwind.config.js, migrated theme to CSS @theme block in index.css
- Replaced @tailwind directives with @import 'tailwindcss'
- Added @custom-variant dark, @utility blocks for custom utilities
- Updated class names across 128 files (shadow-sm → shadow-xs, etc.)
- Removed autoprefixer (built into v4)
- Added migration plan doc

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/library/FolderSidebar.tsx

@@ -162,7 +162,7 @@ function FolderItem({
           <div
             className={cn(
               'absolute right-0 top-full z-10 mt-1 w-40 rounded-md border border-border',
-              'bg-card backdrop-blur-sm py-1 shadow-lg'
+              'bg-card backdrop-blur-xs py-1 shadow-lg'
             )}
           >
             <button
@@ -362,7 +362,7 @@ export function FolderSidebar({
     {/* Mobile backdrop */}
     {mobileOpen && (
       <div
-        className="fixed inset-0 z-40 bg-black/80 backdrop-blur-sm md:hidden"
+        className="fixed inset-0 z-40 bg-black/80 backdrop-blur-xs md:hidden"
         onClick={onMobileClose}
         aria-hidden="true"
       />
@@ -461,7 +461,7 @@ export function FolderSidebar({
       <div
         className={cn(
           'fixed z-50 w-44 rounded-md border border-border',
-          'bg-card backdrop-blur-sm py-1 shadow-lg'
+          'bg-card backdrop-blur-xs py-1 shadow-lg'
         )}
         style={{ left: contextMenu.x, top: contextMenu.y }}
         onClick={(e) => e.stopPropagation()}