feat: add mobile responsiveness, design consistency, and micro-interactions

- Add mobile hamburger menu with slide-out nav drawer (AppLayout)
- Make modals responsive: full-width on mobile, slide-up animation
- Scratchpad becomes full-screen overlay on mobile with backdrop
- Folder sidebar hidden on mobile, opens as slide-over drawer
- Tree editor shows "Desktop Required" gate on mobile
- Stack action buttons vertically on mobile (sessions, detail pages)
- Increase touch targets throughout (buttons, close icons)
- Add CSS animations: fade-in, slide-in-left, scale-in, btn-press
- Add card hover lift effect and consistent border highlights
- Standardize page padding (px-4 py-6 sm:px-6 sm:py-8)
- Responsive headings (text-2xl sm:text-3xl)
- CustomStepModal goes full-screen on mobile
- Tighten auth page spacing on mobile

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

frontend/src/pages/LoginPage.tsx

@@ -38,16 +38,16 @@ export function LoginPage() {
     <div className="flex min-h-screen items-center justify-center bg-background px-4">
       <div className="w-full max-w-md space-y-8">
         <div className="text-center">
-          <div className="mb-6 flex justify-center">
-            <BrandLogo size="lg" />
+          <div className="mb-4 flex justify-center sm:mb-6">
+            <BrandLogo size="lg" className="h-12 w-12 sm:h-16 sm:w-16" />
           </div>
           <h1>
             <BrandWordmark size="lg" />
           </h1>
-          <p className="mt-3 text-lg font-medium text-gradient-brand">
+          <p className="mt-2 text-base font-medium text-gradient-brand sm:mt-3 sm:text-lg">
             Decision Tree Platform
           </p>
-          <p className="mt-2 text-sm text-muted-foreground">
+          <p className="mt-1 text-sm text-muted-foreground sm:mt-2">
             Sign in to your account
           </p>
         </div>
@@ -108,11 +108,11 @@ export function LoginPage() {
               type="submit"
               disabled={isLoading}
               className={cn(
-                'w-full rounded-md px-4 py-2.5 text-sm font-semibold text-white',
+                'w-full rounded-md px-4 py-2.5 text-sm font-semibold text-white btn-press',
                 'bg-gradient-brand hover:bg-gradient-brand-hover',
                 'focus:outline-none focus:ring-2 focus:ring-primary focus:ring-offset-2',
                 'disabled:cursor-not-allowed disabled:opacity-50',
-                'transition-all shadow-lg shadow-primary/20'
+                'shadow-lg shadow-primary/20'
               )}
             >
               {isLoading ? 'Signing in...' : 'Sign in'}