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/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">