feat(chat): share a chat by link — anonymous read-only /shared/<token> page, share/unshare

frontend/history.html

@@ -135,7 +135,9 @@
 
       <!-- Phase 50: the full-width table (AGENTS.md rule 5 — no skinny
            list): Title (the Open link → /?chat=<id>) | Messages |
-           Updated | Actions (Delete, inline two-step confirm).
+           Updated | 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
            hidden and is revealed by a 0-row fetch. The Actions column
            header is visually-hidden — the row buttons carry their own
@@ -148,12 +150,13 @@
               <th scope="col">Title</th>
               <th scope="col">Messages</th>
               <th scope="col">Updated</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="4">No saved chats yet — finish a conversation and press <strong>Save</strong> in the chat.</td>
+              <td colspan="5">No saved chats yet — finish a conversation and press <strong>Save</strong> in the chat.</td>
             </tr>
           </tbody>
         </table>