chore: Tailwind CSS v3 → v4 migration (#99)

* 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>

* chore: switch from @tailwindcss/postcss to @tailwindcss/vite (Phase 2)

- Replaced @tailwindcss/postcss with @tailwindcss/vite plugin
- Deleted postcss.config.js (no longer needed)
- Tailwind now runs as a native Vite plugin for faster HMR

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

* refactor: convert to OKLCH colors, move keyframes into @theme (Phase 3-4)

- Replaced all HSL color indirection with direct OKLCH values in @theme
- Moved all keyframes inside @theme block (v4 pattern)
- Eliminated hsl(var(--x)) double-indirection across 17 component files
- Replaced hsl() inline styles with var(--color-*) theme references
- Cleaned up redundant rdp-* utility blocks
- Fixed @custom-variant dark syntax to use :where()
- Added sidebar/glass/shadow vars as OKLCH in :root

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/LoginPage.tsx

@@ -106,7 +106,7 @@ export function LoginPage() {
                 className={cn(
                   'block w-full rounded-[10px] border border-border bg-card px-3 py-2.5',
                   'text-foreground placeholder:text-muted-foreground',
-                  'focus:border-[rgba(6,182,212,0.3)] focus:outline-none focus:ring-1 focus:ring-primary/20',
+                  'focus:border-[rgba(6,182,212,0.3)] focus:outline-hidden focus:ring-1 focus:ring-primary/20',
                   'transition-colors'
                 )}
                 placeholder="you@example.com"
@@ -127,7 +127,7 @@ export function LoginPage() {
                 className={cn(
                   'block w-full rounded-[10px] border border-border bg-card px-3 py-2.5',
                   'text-foreground placeholder:text-muted-foreground',
-                  'focus:border-[rgba(6,182,212,0.3)] focus:outline-none focus:ring-1 focus:ring-primary/20',
+                  'focus:border-[rgba(6,182,212,0.3)] focus:outline-hidden focus:ring-1 focus:ring-primary/20',
                   'transition-colors'
                 )}
                 placeholder="••••••••••"
@@ -145,8 +145,8 @@ export function LoginPage() {
               disabled={isLoading}
               className={cn(
                 'w-full rounded-[10px] px-4 py-2.5 text-sm font-semibold',
-                'bg-gradient-brand text-[#101114] shadow-lg shadow-primary/20 hover:opacity-90 active:scale-[0.97]',
-                'focus:outline-none focus:ring-2 focus:ring-primary/30 focus:ring-offset-2 focus:ring-offset-background',
+                'bg-gradient-brand text-brand-dark shadow-lg shadow-primary/20 hover:opacity-90 active:scale-[0.97]',
+                'focus:outline-hidden focus:ring-2 focus:ring-primary/30 focus:ring-offset-2 focus:ring-offset-background',
                 'disabled:cursor-not-allowed disabled:opacity-50',
                 'transition-all'
               )}