feat(client): pin node identities on first use — closes 11.5.8

The client verified the node's Ed25519 signature but did not remember which key
it had seen, so a substituted node was caught only by its lack of the GEK.

Trust On First Use: the node's public key is recorded per node_id on the first
successful handshake and compared on every later one. A change is refused
outright — strict, per operator decision. A warning users can click through is
decorative, and this is the SSH known-hosts tradeoff taken deliberately.

Scope, stated honestly: with C6 closed this is defence in depth, not the primary
control. A substituted node already fails the GEK proof. Pinning covers the case
where an attacker HAS the group key — an ex-member, or a leaked GEK — and swaps
the node underneath, which the proof alone cannot distinguish from the real one.

Strict refusal needs an escape hatch or it is a dead end: a node operator who
reinstalls and loses their keystore generates a new pk_node and would otherwise
lock out every member. Settings gains a "Node identities" section showing the
pin count and clearing them, with copy telling the user to verify out of band
first. Also exposed as MeshBayTransport.clearNodePin() for the native client.

Tests: hub+common green; all five static JS files syntax-checked.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/app.js b/packages/meshbay-hub/src/meshbay_hub/static/app.js
index dee47ec..a006dfe 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/app.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/app.js
@@ -1978,6 +1978,15 @@ function SettingsPage({ user, theme, onThemeChange, groups }) {
   const [currentNodeKey, setCurrentNodeKey] = useState(null);
   const [nodeKeyStatus, setNodeKeyStatus] = useState('');
   const [nodeKeyLoading, setNodeKeyLoading] = useState(false);
+  const [pinCount, setPinCount] = useState(
+    () => (window.MeshBayTransport?.pinnedNodeCount?.() ?? 0));
+
+  // 11.5.8: node identity pins are refused strictly on change, so users need a
+  // deliberate way to accept a legitimate rotation (operator reinstalled a node).
+  const clearPins = useCallback(() => {
+    window.MeshBayTransport?.clearNodePin?.();
+    setPinCount(window.MeshBayTransport?.pinnedNodeCount?.() ?? 0);
+  }, []);
 
   useEffect(() => {
     hubFetch(`/v1/users/${user.username}/pubkeys`, { token: user.token })
@@ -2069,6 +2078,17 @@ function SettingsPage({ user, theme, onThemeChange, groups }) {
       </div>
 
       <div class="settings-section">
+        <h3 class="settings-heading">${t('settings.node_pins')}</h3>
+        <p class="settings-hint">${t('settings.node_pins_hint')}</p>
+        <div class="settings-row">
+          <span class="settings-label">${t('settings.node_pins_count', { n: pinCount })}</span>
+          <button class="btn-secondary" onClick=${clearPins} disabled=${pinCount === 0}>
+            ${t('settings.node_pins_clear')}
+          </button>
+        </div>
+      </div>
+
+      <div class="settings-section">
         <h3 class="settings-heading">${t('settings.appearance')}</h3>
         <div class="settings-row">
           <span class="settings-label">${t('settings.theme')}</span>