feat(fof-layer4): Tauri commands + Settings "Rotate my vouch key" UI

Three new Tauri commands surface Layer 4 to the frontend:

- rotate_v_me() -> { newEpoch }: generates next V_me epoch +
  republishes bio. Old epoch retained in vouch_keys_own; existing
  vouchees receive the new key on their next bio-scan.
- cascade_revoke_v_me_epoch(retired_epoch, reason_code) ->
  { postsRevoked }: bulk per-post revocation across every author
  post that sealed slots under (self, retired_epoch). Useful as a
  follow-up after rotate_v_me when the author wants to actively
  cut off comment access on old posts.
- key_burn_post_slot(post_id_hex, slot_index, new_v_x_hex): the
  leaked-V_me primitive. Re-seals one slot under a different V_x.

Frontend (Settings → Vouches):
- New "Rotate my vouch key" button + status pill below the Given /
  Received lists.
- Confirmation prompt explains the grandfather-by-default semantics:
  "old posts remain readable to anyone who held the old key — cascade-
  revoke separately if you want to cut off old-content access."
- Wired once per settings-tab activation.

Cascade-revoke and key-burn surfaces aren't visualized yet (require
per-post selection UI); the Tauri commands are available for follow-up
UI work or scripting via the desktop dev console.

Workspace builds clean; 148 tests pass.

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

frontend/index.html

@@ -213,6 +213,11 @@
             <div id="vouches-received-list"></div>
           </div>
         </div>
+        <div style="margin-top:0.75rem;padding-top:0.75rem;border-top:1px solid var(--border)">
+          <p class="empty-hint" style="margin-bottom:0.4rem">Rotate your vouch key (V_me) to issue a fresh key to all your current vouchees. Old posts remain readable by anyone who held the old key &mdash; use "Cascade revoke" afterward to actively cut off comment access to your old posts.</p>
+          <button id="rotate-v-me-btn" class="btn btn-ghost btn-sm">Rotate my vouch key</button>
+          <span id="rotate-v-me-status" class="empty-hint" style="margin-left:0.5rem;font-size:0.8rem"></span>
+        </div>
       </div>
 
       <div class="section-card" style="text-align:center">