feat: sidebar nav (default), group member management, per-customer ticketing identity mapping

- Navigation: new per-user setting (default "side") renders the page links as a
  left sidebar while keeping theme/bell/avatar/logout in the top-right; falls
  back to a top bar under 760px or when set to "top". Profile selector added.
- Group conversations: record a creatorId; the creator gets a "Manage members"
  button to add/remove participants (new GET/POST/DELETE members endpoints,
  creator-only). Existing groups backfilled.
- Customer ticketing: admin can map each assigned consultant to their username
  in that customer's ticketing system. Per-customer mapping takes precedence
  over the consultant's global ticketingIdentities during sync.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/templates/profile.html

@@ -65,6 +65,13 @@
       <option value="sunset" {{if eq .User.Settings.Theme "sunset"}}selected{{end}}>Sunset (vaporwave)</option>
     </select>
   </div>
+  <div class="field">
+    <label for="p-nav">Navigation</label>
+    <select id="p-nav">
+      <option value="side" {{if ne .User.Settings.NavLayout "top"}}selected{{end}}>Sidebar (left)</option>
+      <option value="top" {{if eq .User.Settings.NavLayout "top"}}selected{{end}}>Top bar</option>
+    </select>
+  </div>
 
   <button class="btn primary" type="submit">Save profile</button>
 </form>