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/components/NewItemDialog.tsx

@@ -15,7 +15,7 @@ interface NewItemDialogProps {
 
 export default function NewItemDialog({ onScannerClick, onAddItemClick }: NewItemDialogProps) {
   return (
-    <div className="flex flex-col items-center py-8 text-center gap-6">
+    <div className="flex flex-col items-center py-3 md:py-4 text-center gap-3 md:gap-4">
       <button
         onClick={onScannerClick}
         className="w-24 h-24 rounded-full bg-primary/20 hover:bg-primary/30 border-2 border-primary border-dashed flex items-center justify-center group transition-all"
@@ -23,10 +23,10 @@ export default function NewItemDialog({ onScannerClick, onAddItemClick }: NewIte
         <Smartphone className="w-10 h-10 text-primary group-hover:scale-110 transition-transform" />
       </button>
 
-      <div className="w-full flex justify-center mt-4">
+      <div className="w-full flex justify-center">
         <button
           onClick={onAddItemClick}
-          className="w-full flex flex-col items-center justify-center p-8 rounded-[2rem] bg-indigo-500/5 border border-indigo-500/20 group hover:border-indigo-500/50 transition-all font-normal text-indigo-400 gap-4"
+          className="w-full flex flex-col items-center justify-center p-4 md:p-6 rounded-[2rem] bg-indigo-500/5 border border-indigo-500/20 group hover:border-indigo-500/50 transition-all font-normal text-indigo-400 gap-2 md:gap-3"
         >
           <div className="p-4 bg-indigo-500/10 rounded-2xl group-hover:scale-110 transition-transform shadow-lg shadow-indigo-500/10">
             <Sparkles size={32} />