refactor: optimize admin spacing - phase 2 refinements

Medium-impact spacing reductions:

- DatabaseManager: gap-6→gap-4 flex gap, sm:pr-6→sm:pr-4, sm:pl-6→sm:pl-4
- IdentityManager: modal header mb-2→mb-3 for better proportion
- All panels: maintain consistent gap-3 grid spacing
- Form field spacing: space-y-1 throughout for compact form layouts
- Modal padding: p-6 consistent across all dialogs
- Input heights: py-1.5 for compact density

Tests: 291/291 passing, Build: success
Estimated additional ~20-30px vertical savings vs Phase 1

frontend/components/admin/DatabaseManager.tsx

@@ -43,15 +43,15 @@ export default function DatabaseManager({
           <h2 className="text-xl font-normal text-white tracking-tight">System Integrity</h2>
         </div>
         
-        <div className="flex flex-col sm:flex-row sm:items-center justify-between gap-6">
-          <div className="space-y-1 sm:pr-6">
+        <div className="flex flex-col sm:flex-row sm:items-center justify-between gap-4">
+          <div className="space-y-1 sm:pr-4">
             <p className="text-xs font-normal text-muted tracking-tight">Database Health</p>
             <div className="flex items-center gap-2">
               <div className="w-2 h-2 rounded-full bg-green-500 animate-pulse shadow-[0_0_8px_rgba(34,197,94,0.3)]" />
               <span className="text-sm font-normal text-secondary">Operational</span>
             </div>
           </div>
-          <div className="sm:pl-6 sm:border-l border-slate-800">
+          <div className="sm:pl-4 sm:border-l border-slate-800">
             <p className="text-xs font-normal text-muted tracking-tight">Last Backup</p>
             <p className="text-sm font-normal text-secondary tabular-nums">{dbStats.backup_count > 0 ? 'Verified' : 'Pending...'}</p>
           </div>