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/hooks/useTheme.ts

@@ -0,0 +1,59 @@
+import { useCallback, useEffect, useState } from 'react';
+
+export type Theme = 'light' | 'dark' | 'system';
+
+const STORAGE_KEY = 'mimic-theme';
+
+function resolveTheme(theme: Theme): 'light' | 'dark' {
+  if (theme === 'system') {
+    return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+  }
+  return theme;
+}
+
+function applyTheme(theme: Theme) {
+  const resolved = resolveTheme(theme);
+  document.documentElement.classList.toggle('dark', resolved === 'dark');
+}
+
+function readStoredTheme(): Theme {
+  try {
+    const stored = localStorage.getItem(STORAGE_KEY);
+    if (stored === 'light' || stored === 'dark' || stored === 'system') return stored;
+  } catch {
+    // localStorage unavailable
+  }
+  return 'system';
+}
+
+export function useTheme() {
+  const [theme, setThemeState] = useState<Theme>(readStoredTheme);
+
+  useEffect(() => {
+    applyTheme(theme);
+  }, [theme]);
+
+  // Track system preference changes when theme === 'system'
+  useEffect(() => {
+    if (theme !== 'system') return;
+    const mq = window.matchMedia('(prefers-color-scheme: dark)');
+    const handler = () => applyTheme('system');
+    mq.addEventListener('change', handler);
+    return () => mq.removeEventListener('change', handler);
+  }, [theme]);
+
+  const setTheme = useCallback((next: Theme) => {
+    try {
+      localStorage.setItem(STORAGE_KEY, next);
+    } catch {
+      // ignore
+    }
+    setThemeState(next);
+  }, []);
+
+  const cycleTheme = useCallback(() => {
+    setTheme(theme === 'light' ? 'dark' : theme === 'dark' ? 'system' : 'light');
+  }, [theme, setTheme]);
+
+  return { theme, setTheme, cycleTheme };
+}