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/admin/page.tsx

@@ -25,8 +25,8 @@ export default function AdminPage() {
 
   return (
     <PageShell>
-      <main data-testid="admin-page" className="p-3 md:p-8 max-w-7xl mx-auto space-y-6 md:space-y-10 mb-20">
-        <header className="flex items-center gap-4 mb-8 md:mb-12">
+      <main data-testid="admin-page" className="p-3 md:p-8 max-w-7xl mx-auto space-y-3 md:space-y-6 mb-20">
+        <header className="flex items-center gap-4 mb-4 md:mb-6">
           <div className="p-3 md:p-4 bg-indigo-500/10 rounded-2xl text-indigo-400 border border-indigo-500/20 shadow-xl shadow-indigo-500/5">
             <Shield size={28} className="md:w-8 md:h-8" />
           </div>
@@ -44,9 +44,9 @@ export default function AdminPage() {
           </button>
         </header>
 
-        <div className="grid lg:grid-cols-2 gap-6 md:gap-8 items-start">
+        <div className="grid lg:grid-cols-2 gap-3 md:gap-4 items-start">
           {/* Left Column: Identity & Integrity */}
-          <section className="space-y-6 md:space-y-8">
+          <section className="space-y-3 md:space-y-4">
             <DatabaseManager data-testid="admin-tab-database"
               dbStats={admin.dbStats}
               isBackingUp={admin.isBackingUp}
@@ -72,7 +72,7 @@ export default function AdminPage() {
           </section>
 
           {/* Right Column: Infrastructure */}
-          <section className="space-y-6 md:space-y-8">
+          <section className="space-y-3 md:space-y-4">
             <LdapManager data-testid="admin-tab-ldap"
               ldapConfig={admin.ldapConfig}
               setLdapConfig={admin.setLdapConfig}