fix: redesign buttons — depth, outline contrast, press effect

- button.tsx: primary gets gradient + shadow + hover glow + -translate-y-px
  lift; outline gets border-2 for visibility on dark bg; all variants
  get active:scale-[0.96] press feel; lg size h-12 for auth forms
- Remove hardcoded bg-[var(--accent)] overrides from Login, Register,
  Feed, Profile, Upload — variants now handle all coloring

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

client/src/pages/LoginPage.tsx

@@ -71,7 +71,7 @@ export default function LoginPage() {
             <Button
               type="submit"
               disabled={loading}
-              className="w-full h-12 rounded-xl text-sm font-semibold bg-[var(--accent)] hover:bg-[var(--accent-hover)] text-white transition-all active:scale-[0.98] shadow-sm"
+              size="lg" className="w-full"
             >
               {loading ? (
                 <span className="flex items-center gap-2">