fix: critical mobile viewport fixes - login, modals, page constraints

- Remove min-h-screen or make responsive (md:min-h-screen for desktop only)
- Login modal: p-8 → p-4 md:p-6, space-y-8 → space-y-3 md:space-y-4
- NewItemDialog: responsive spacing p-4 md:p-6, gaps 3 md:gap-4
- StockAdjustmentPanel: p-6 → p-4 md:p-6, gaps/spacing reduced for mobile
- All container padding and gaps now follow mobile-first pattern
- Fixes viewport overflow on 375px portrait mode
- Tests: 291/291 passing

frontend/app/login/page.tsx

@@ -79,10 +79,10 @@ export default function LoginPage() {
   if (!mounted) return null;
 
   return (
-    <div data-testid="identity-check-overlay" className="min-h-screen bg-background flex items-center justify-center p-4">
+    <div data-testid="identity-check-overlay" className="bg-background flex items-center justify-center p-4 md:min-h-screen">
       <Toaster position="top-center" />
-      
-      <div className="bg-surface border border-slate-800 rounded-3xl p-8 max-w-sm w-full shadow-2xl space-y-8 animate-in fade-in zoom-in duration-500">
+
+      <div className="bg-surface border border-slate-800 rounded-3xl p-4 md:p-6 max-w-sm w-full shadow-2xl space-y-3 md:space-y-4 animate-in fade-in zoom-in duration-500">
         <div className="text-center space-y-2">
           <div className="w-16 h-16 bg-primary/10 text-primary rounded-2xl flex items-center justify-center mx-auto mb-4 border border-primary/20">
             <User size={32} />
@@ -91,7 +91,7 @@ export default function LoginPage() {
           <p className="text-muted text-sm">Select operator profile or use direct login</p>
         </div>
         
-        <div data-testid="user-list" className="grid gap-3">
+        <div data-testid="user-list" className="grid gap-2 md:gap-3">
           {!selectedUserForLogin && !isEnterprise ? (
             <>
               {users.length > 0 ? (