Phase 2c: remove audience + PostPush + PostNotification + AudienceRequest/Response

v0.6.2 wire fork: every persona-identifying direct push is gone. Public posts
propagate only through the CDN (pull + header-diff neighbor propagation).
Encrypted posts propagate only through pull with merged author-or-recipient
match. There is no remaining sender→recipient traffic correlation signal on
the wire for content.

Protocol (network-breaking):
- Retire MessageType 0x42 (PostNotification), 0x43 (PostPush),
  0x44 (AudienceRequest), 0x45 (AudienceResponse). Their payload structs are
  deleted along with the handlers and senders.
- SocialDisconnectNotice (0x71) / SocialAddressUpdate (0x70) sender
  functions targeting audience are deleted; the existing handlers stay
  (both already dead code on the send side).

Core removals:
- `push_to_audience`, `notify_post`, `push_delete`,
  `push_disconnect_to_audience`, `push_address_update_to_audience`,
  `send_audience_request`, `send_audience_response`, `send_to_audience` —
  all gone from network.rs.
- `handle_post_notification` removed from connection.rs.
- `request_audience`, `approve_audience`, `deny_audience`,
  `remove_audience`, `list_audience_members`, `list_audience` removed from
  Node.
- `audience_pushed` step removed from post creation.
- `AudienceDirection`, `AudienceStatus`, `AudienceRecord`,
  `AudienceApprovalMode` removed from types.
- Storage: `store_audience`, `list_audience`, `list_audience_members`,
  `remove_audience`, `row_to_audience_record`, `audience_crud` test, the
  `audience` CREATE TABLE, and the audience-dependent social route rebuild
  branch all removed. Upgraded DBs retain the orphan `audience` table;
  nothing touches it.

Follow-on cleanups:
- `SocialRelation::Audience` + `::Mutual` collapsed into just `Follow`.
  The Display/FromStr impl accepts legacy "audience"/"mutual" strings from
  pre-v0.6.2 DBs and maps them to Follow.
- Blob-eviction priority function drops the audience factor; relationship
  is now own-author vs followed vs other. Tests updated accordingly.
- `CommentPermission::AudienceOnly` → `FollowersOnly`. Check uses the
  author's public follows (`list_public_follows`) rather than a separate
  audience table. `ModerationMode::AudienceOnly` similarly renamed.
- Follow/unfollow routines simplified: no audience downgrade logic;
  unfollow removes the social route entirely.

UI:
- CLI: `audience*` commands removed.
- Tauri: `AudienceDto`, `list_audience`, `list_audience_outbound`,
  `request_audience`, `approve_audience`, `remove_audience` commands
  removed from invoke_handler. Frontend: audience panel and audience/mutual
  badges removed; compose permission dropdown shows "Followers" instead of
  "Audience"; `loadAudience` is a no-op stub that hides any leftover DOM.

Tests: 111 / 111 core tests pass.

Breaking change: v0.6.2 nodes won't interoperate with v0.6.1 for delete
propagation, visibility updates, direct post push, post notifications, or
audience requests. Upgrade both ends.

frontend/index.html

@@ -96,7 +96,7 @@
               <select id="circle-select" class="hidden"></select>
               <select id="comment-perm-select" title="Comment permission">
                 <option value="public">Comments: All</option>
-                <option value="audience_only">Comments: Audience</option>
+                <option value="followers_only">Comments: Followers</option>
                 <option value="none">Comments: Off</option>
               </select>
               <select id="react-perm-select" title="React permission">
@@ -132,16 +132,7 @@
         </div>
       </div>
 
-      <div class="section-card">
-        <h3>Audience</h3>
-        <p class="empty-hint">People who receive your public posts.</p>
-        <h4 class="subsection-title">Pending Requests</h4>
-        <div id="audience-pending-list"></div>
-        <h4 class="subsection-title">Approved</h4>
-        <div id="audience-approved-list"></div>
-      </div>
-
-      <div class="section-card" style="display:flex;gap:0.5rem;flex-wrap:wrap">
+<div class="section-card" style="display:flex;gap:0.5rem;flex-wrap:wrap">
         <button id="share-details-btn" class="btn btn-ghost btn-sm">Share my details</button>
         <button id="connect-toggle" class="btn btn-ghost btn-sm">Add peer manually</button>
         <div id="connect-body" class="hidden">