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/IdentityManager.tsx

@@ -87,7 +87,7 @@ export default function IdentityManager({
       {editingUser && (
         <div className="fixed inset-0 z-[100] flex items-center justify-center p-4 bg-background/80  animate-in fade-in duration-200">
           <div className="bg-surface border border-slate-800 rounded-[2.5rem] p-6 w-full max-w-md shadow-2xl">
-            <div className="flex justify-between items-center mb-2">
+            <div className="flex justify-between items-center mb-3">
               <h3 className="text-xl font-normal text-white tracking-tight">Edit Identity</h3>
               <button
                 onClick={() => setEditingUser(null)}