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/pages/MyAnalyticsPage.tsx

@@ -90,7 +90,7 @@ export default function MyAnalyticsPage() {
           <select
             value={period}
             onChange={(e) => setPeriod(e.target.value as AnalyticsPeriod)}
-            className="rounded-lg border border-border bg-card px-3 py-1.5 text-sm text-foreground focus:outline-none focus:ring-1 focus:ring-primary/20"
+            className="rounded-lg border border-border bg-card px-3 py-1.5 text-sm text-foreground focus:outline-hidden focus:ring-1 focus:ring-primary/20"
           >
             {PERIOD_OPTIONS.map((opt) => (
               <option key={opt.value} value={opt.value}>
@@ -298,7 +298,7 @@ export default function MyAnalyticsPage() {
                     <div className="flex items-center justify-between mb-1">
                       <div className="flex items-center gap-2">
                         <div
-                          className="h-2.5 w-2.5 rounded-full flex-shrink-0"
+                          className="h-2.5 w-2.5 rounded-full shrink-0"
                           style={{
                             backgroundColor:
                               OUTCOME_COLORS[outcome] ?? '#94a3b8',