feat(fof-layer2): Tauri commands + frontend compose/comment routing

Three new Tauri commands surface Layer 2 to the frontend:

- create_post_with_fof_comments(content) → { postId }
  Builds the FoF gating block from the default persona's keyring and
  publishes a Mode 2 post (public body, FoF-gated comments).
- comment_on_fof_post(post_id_hex, body)
  Backwards-compat wrapper: comment_on_post now auto-detects FoF
  gating on the parent post and dispatches internally. Existing
  comment-send UI works unchanged.
- revoke_fof_commenter(post_id_hex, pub_x_index, reason_code)
  Wraps the Node helper. UI for per-comment revocation can hang off
  this without further Node changes.

Node::comment_on_post now branches on parent post fof_gating presence
— this is the central routing point so neither the frontend nor
existing comment-send code needs to know about FoF specifics. FoF
posts get FoF comments automatically; non-FoF posts get the legacy
path. Same Tauri command, same frontend handler.

Frontend:
- Compose: comment-perm-select gains a "Friends of Friends" option.
  When picked, compose dispatches to create_post_with_fof_comments
  instead of the standard create_post. Standard set_comment_policy
  diff fires after so receivers' four-check accept rule activates.
- Attachments + non-default-persona FoF posts are out-of-v1; compose
  reports + aborts rather than silently producing a non-FoF post.

Layer 2 backend + minimal-viable UI complete. Granular per-comment
revoke UI and access-grant UI deferred — Node + Tauri primitives
exist; the surfaces can be added without further crypto work.

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

frontend/app.js

@@ -2629,8 +2629,29 @@ async function doPost() {
             }
         }
 
+        const commentPerm = document.getElementById('comment-perm-select').value;
+        const reactPerm = document.getElementById('react-perm-select').value;
+
         let result;
-        if (selectedFiles.length > 0) {
+        if (commentPerm === 'friends_of_friends') {
+            // FoF Layer 2: body is still public (Mode 2) but the post
+            // carries a fof_gating block built from the author's
+            // keyring. Routed through a dedicated command because the
+            // gating block is signed at publish time (can't be added
+            // via SetPolicy after the fact).
+            // Attachments + non-default-persona FoF posts are not yet
+            // supported by the v1 command — fall through to the
+            // standard path with a warning if either applies.
+            if (selectedFiles.length > 0 || params.postingIdHex) {
+                toast('FoF posts with attachments or non-default persona require Mode 1 (later).');
+                postBtn.disabled = false;
+                return;
+            }
+            const created = await invoke('create_post_with_fof_comments', {
+                content: params.content,
+            });
+            result = { id: created.postId };
+        } else if (selectedFiles.length > 0) {
             // Convert ArrayBuffers to base64 strings
             const files = selectedFiles.map(f => {
                 const bytes = new Uint8Array(f.data);
@@ -2644,9 +2665,9 @@ async function doPost() {
             result = await invoke('create_post', params);
         }
 
-        // Set engagement policy if non-default
-        const commentPerm = document.getElementById('comment-perm-select').value;
-        const reactPerm = document.getElementById('react-perm-select').value;
+        // Set engagement policy if non-default (FoF posts also publish
+        // the policy diff so receivers route the comment-receive path
+        // through the FoF four-check verify gate).
         if ((commentPerm !== 'public' || reactPerm !== 'both') && result && result.id) {
             try {
                 await invoke('set_comment_policy', {

frontend/index.html

@@ -104,6 +104,7 @@
               <select id="comment-perm-select" title="Comment permission">
                 <option value="public">Comments: All</option>
                 <option value="followers_only">Comments: Followers</option>
+                <option value="friends_of_friends">Comments: Friends of Friends</option>
                 <option value="none">Comments: Off</option>
               </select>
               <select id="react-perm-select" title="React permission">