feat(chat): invalidate saved chats on sources sync — versioned stamps, stale marker, Regenerate against the new index

frontend/history.html

@@ -135,7 +135,10 @@
 
       <!-- Phase 50: the full-width table (AGENTS.md rule 5 — no skinny
            list): Title (the Open link → /?chat=<id>) | Messages |
-           Updated | Share (phase 51: Create link / Copy / Unshare —
+           Updated | Stale (phase 53: the READ-ONLY staleness marker —
+           the rose pill when the row predates the last KB-changing
+           sync; the Regenerate action lives on the chat-page banner,
+           task 05) | Share (phase 51: Create link / Copy / Unshare —
            the row's share_url comes from GET /api/chats itself, no
            second fetch) | Actions (Delete, inline two-step confirm).
            history.js fills #history-tbody; #history-empty-row ships
@@ -150,13 +153,14 @@
               <th scope="col">Title</th>
               <th scope="col">Messages</th>
               <th scope="col">Updated</th>
+              <th scope="col">Stale</th>
               <th scope="col">Share</th>
               <th scope="col"><span class="visually-hidden">Actions</span></th>
             </tr>
           </thead>
           <tbody id="history-tbody">
             <tr class="history-empty-row" id="history-empty-row" hidden>
-              <td colspan="5">No saved chats yet — finish a conversation and press <strong>Save</strong> in the chat.</td>
+              <td colspan="6">No saved chats yet — finish a conversation and press <strong>Save</strong> in the chat.</td>
             </tr>
           </tbody>
         </table>