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/messages.html

@@ -11,7 +11,10 @@
   <section class="chat-main card">
     <div class="spread">
       <h2 id="chat-title">Select a conversation</h2>
-      <span class="muted" id="typing-indicator" aria-live="polite"></span>
+      <span style="display:flex;align-items:center;gap:10px">
+        <span class="muted" id="typing-indicator" aria-live="polite"></span>
+        <button class="btn small" id="chat-members" type="button" hidden>Manage members</button>
+      </span>
     </div>
     <div id="chat-messages" class="chat-messages" aria-live="polite"></div>
     <form id="chat-form" hidden>
@@ -66,6 +69,21 @@
   </form>
 </dialog>
 
+<dialog id="members-dialog">
+  <div class="stack" style="min-width:420px">
+    <div class="spread">
+      <h2 style="margin:0">Group members</h2>
+      <button class="btn small" type="button" id="mm-close">Close</button>
+    </div>
+    <ul id="mm-list" class="conv-list"></ul>
+    <div class="field" id="mm-add-wrap">
+      <label for="mm-search">Add someone</label>
+      <input type="search" id="mm-search" placeholder="Search people…">
+      <div id="mm-results" role="listbox" aria-label="Search results"></div>
+    </div>
+  </div>
+</dialog>
+
 <dialog id="lightbox" class="lightbox">
   <img id="lightbox-img" alt="">
 </dialog>