feat(ui): reviewable security banner with suspected-text + Allow/Block

Banner previously always rendered "Session terminated" — one-way. Now
when security_event.reviewable=true:

- Title switches to "Review suspected injection"
- Subtitle explains the decision ("allow to continue, block to end")
- Expandable details auto-open so the user sees context immediately
- Suspected text excerpt rendered in a mono pre block, scrollable,
  capped at 500 chars server-side
- Per-layer confidence scores (which layer fired, how confident)
- Action row with red [Block session] + neutral [Allow and continue]
- Click posts to /security-decision, banner hides, sidebar-agent
  sees the file and resumes or kills within one poll cycle

Existing hard-block banner (terminated session, canary leaks) unchanged.

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

extension/sidepanel.html

@@ -47,6 +47,12 @@
     <div class="security-banner-details" id="security-banner-details" hidden>
       <div class="security-banner-section-label">SECURITY LAYERS</div>
       <div class="security-banner-layers" id="security-banner-layers"></div>
+      <div class="security-banner-section-label" id="security-banner-suspect-label" hidden>SUSPECTED TEXT</div>
+      <pre class="security-banner-suspect" id="security-banner-suspect" hidden></pre>
+    </div>
+    <div class="security-banner-actions" id="security-banner-actions" hidden>
+      <button type="button" class="security-banner-btn security-banner-btn-block" id="security-banner-btn-block">Block session</button>
+      <button type="button" class="security-banner-btn security-banner-btn-allow" id="security-banner-btn-allow">Allow and continue</button>
     </div>
   </div>