feat(m4): frontend MitreTagPicker + /mitre showcase page

- lib/mitre.ts: shared types (MitreTactic, Technique, Subtechnique, MitreTag
  kind/id/external_id/name) + TanStack query keys.
- components/MitreTagPicker.tsx: three-column controlled picker (tactic →
  technique → subtechnique), multi-select with chip-removal, autocomplete on
  each column, ARIA labels for screen readers. Returns MitreTag[] via
  value/onChange — drop-in for M5 template forms.
- pages/MitrePage.tsx: status card (version, source URL, last_sync), admin-
  gated Trigger Sync button with success/error alerts, picker showcase, JSON
  preview of the current selection.
- Layout adds MITRE nav link for any logged-in user; App.tsx adds the
  /mitre route under RequireAuth. HomePage roadmap bumped to next: M5
  templates.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/mitre.ts

@@ -0,0 +1,61 @@
+/** Shared types + query keys for MITRE ATT&CK browsing. */
+
+export interface MitreTactic {
+  id: string;
+  external_id: string;
+  short_name: string;
+  name: string;
+  description: string | null;
+  url: string | null;
+}
+
+export interface MitreTechnique {
+  id: string;
+  external_id: string;
+  name: string;
+  description: string | null;
+  url: string | null;
+  tactics: Array<{ external_id: string; name: string }>;
+}
+
+export interface MitreSubtechnique {
+  id: string;
+  external_id: string;
+  name: string;
+  description: string | null;
+  url: string | null;
+  technique_id: string;
+}
+
+export interface Paginated<T> {
+  items: T[];
+  total: number;
+  limit: number;
+  offset: number;
+}
+
+export interface MitreStatus {
+  last_sync: string | null;
+  version: string | null;
+  source_url: string | null;
+  default_url: string;
+  default_version: string;
+}
+
+export type MitreTagKind = 'tactic' | 'technique' | 'subtechnique';
+
+export interface MitreTag {
+  kind: MitreTagKind;
+  id: string;
+  external_id: string;
+  name: string;
+}
+
+export const mitreKeys = {
+  status: ['mitre', 'status'] as const,
+  tactics: (q?: string) => ['mitre', 'tactics', q ?? ''] as const,
+  techniques: (tactic?: string, q?: string) =>
+    ['mitre', 'techniques', tactic ?? '', q ?? ''] as const,
+  subtechniques: (technique?: string, q?: string) =>
+    ['mitre', 'subtechniques', technique ?? '', q ?? ''] as const,
+};