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/AdminOverlay.tsx

@@ -115,7 +115,7 @@ export default function AdminOverlay({
       />
       <div data-testid="admin-overlay" className="fixed inset-0 z-50 bg-background/60 animate-in fade-in duration-300">
       <div className="absolute inset-y-0 right-0 w-full max-w-md sm:max-w-lg md:max-w-md bg-surface border-l border-slate-800 shadow-2xl flex flex-col animate-in slide-in-from-right duration-500">
-        <div className="p-6 border-b border-slate-800 flex items-center justify-between">
+        <div className="p-3 md:p-6 border-b border-slate-800 flex items-center justify-between">
           <div className="flex items-center gap-3">
             <div className="p-2 bg-slate-800 rounded-xl text-primary">
               <Shield size={20} />
@@ -132,8 +132,8 @@ export default function AdminOverlay({
           </button>
         </div>
         
-        <div className="flex-1 overflow-y-auto p-6 space-y-8">
-          <section className="space-y-4">
+        <div className="flex-1 overflow-y-auto p-3 md:p-6 space-y-3 md:space-y-4">
+          <section className="space-y-2 md:space-y-3">
             <div className="flex items-center justify-between">
               <div>
                 <h3 className="text-sm font-normal text-secondary">User Management</h3>