feat(fof-layer1): Tauri commands + frontend UI for vouches

Node helpers (crates/core/src/node.rs):
- vouch_for_peer(target): derives target X25519 pub from NodeId,
  inserts into own_vouch_targets, republishes bio post so the new
  VouchGrantBatch propagates to the receiver via CDN.
- revoke_vouch_and_rotate(target): per Scott's design, revocation IS
  the rotation primitive. Marks target current=0, generates new V_me
  epoch in vouch_keys_own (prior retained, Layer 4 receiver-chain),
  republishes bio. Revoked persona retains old V_me → grandfathered
  access to old content; locked out of new content sealed under V_new.
- list_vouches_given / list_vouches_received: enriched with display
  names via resolve_display_name.

Tauri commands (crates/tauri-app/src/lib.rs):
- vouch_for_peer, revoke_vouch_for_peer (single-action commands)
- list_vouches_given, list_vouches_received (DTOs with camelCase)
- All registered in the generate_handler! list.

Frontend (frontend/index.html + app.js):
- New "Vouches" section in Settings tab with side-by-side Given /
  Received lists. Per-row Revoke button on Given entries with confirm
  prompt explaining the rotation semantics.
- Bio modal gains Vouch / Revoke Vouch button next to Follow/Unfollow.
  State derived from list_vouches_given on modal open.
- loadVouches wired into the settings-tab activation handler.

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

frontend/index.html

@@ -198,6 +198,21 @@
         <div id="ignored-list" style="text-align:left"></div>
       </div>
 
+      <div class="section-card" style="text-align:center">
+        <h3 style="margin-bottom:0.25rem">Vouches</h3>
+        <p class="empty-hint" style="margin-bottom:0.5rem">Vouches you've given let those friends read your Friend-of-Friend posts. Vouches you've received unlock the posts of those who vouched for you. Revoking rotates your vouch key &mdash; the revoked friend keeps access to your existing posts, but not future ones.</p>
+        <div style="display:flex;gap:1rem;text-align:left">
+          <div style="flex:1;min-width:0">
+            <h4 style="margin:0 0 0.4rem;font-size:0.9rem">Given</h4>
+            <div id="vouches-given-list"></div>
+          </div>
+          <div style="flex:1;min-width:0">
+            <h4 style="margin:0 0 0.4rem;font-size:0.9rem">Received</h4>
+            <div id="vouches-received-list"></div>
+          </div>
+        </div>
+      </div>
+
       <div class="section-card" style="text-align:center">
         <h3 style="margin-bottom:0.25rem">Updates</h3>
         <p class="empty-hint" style="margin-bottom:0.5rem">Network-wide release announcements are signed by the bootstrap anchor and arrive via the CDN. Choose which channel to follow.</p>