feat(frontend): sprint 4 — dark mode + matrix overhaul + tactic selection + done read-only + UI polish

US-17: fix duplicate "Create engagement" button, icon conventions (Save/RotateCcw/Grid2x2), UsersAdminPage form baseline alignment
US-18: done status fully read-only + Reopen button (done → review_required) for all roles
US-19: invalidate engagement queries on simulation PATCH/transition for auto-status propagation
US-20: MitreMatrixModal rewritten — CSS grid 12-column layout, no horizontal scroll, attack.mitre.org compact look
US-21: tactic header clickable in matrix, tactic chips (MitreTacticTag) in field, single atomic PATCH with technique_ids + tactic_ids
US-22: MitreTechniquesField chips-only area + inline search input + matrix icon button; chips show ID-only (name in title=)
US-23: useTheme hook — 3-state light/dark/system, CSS variables, Tailwind darkMode class, localStorage persistence

92/92 tests passing, typecheck and lint clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/UsersAdminPage.tsx

@@ -110,7 +110,7 @@ export function UsersAdminPage(): JSX.Element {
 
       <section className="card-product flex flex-col gap-md">
         <h2 className="text-[20px] font-medium">Create account</h2>
-        <form onSubmit={onCreate} className="grid grid-cols-1 md:grid-cols-4 gap-md items-start">
+        <form onSubmit={onCreate} className="grid grid-cols-1 md:grid-cols-4 gap-md items-end">
           <FormField label="Username" htmlFor="new-username" required>
             <TextInput
               id="new-username"
@@ -137,7 +137,7 @@ export function UsersAdminPage(): JSX.Element {
               options={ROLE_OPTIONS}
             />
           </FormField>
-          <div className="self-end">
+          <div>
             <button type="submit" className="btn-primary w-full" disabled={createMutation.isPending}>
               {createMutation.isPending ? 'Creating…' : 'Create'}
             </button>