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/pages/HomePage.tsx

@@ -61,7 +61,7 @@ export function HomePage() {
           <span className="text-purple">Purple Team Platform</span>
         </h1>
         <p className="font-mono text-sm font-light text-text-dim mt-2">
-          Collaborative red &amp; blue test orchestration — M3 milestone (RBAC)
+          Collaborative red &amp; blue test orchestration — M4 milestone (MITRE ATT&amp;CK)
         </p>
       </header>
       <SectionHeader
@@ -141,9 +141,9 @@ export function HomePage() {
 
         <Card accent="purple" title="Roadmap" sub="14 milestones">
           <p>
-            M0 + M1 + M2 + M3 done. Next:{' '}
+            M0 + M1 + M2 + M3 + M4 done. Next:{' '}
             <code className="accent-fill-cyan px-2 py-[2px] rounded-sm font-mono text-4xs">
-              M4 — MITRE ATT&amp;CK
+              M5 — Test &amp; scenario templates
             </code>
             .
           </p>