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/admin.html

@@ -84,6 +84,11 @@
         <select id="c-consultants" multiple size="4"></select>
         <p class="hint">Hold Ctrl/Cmd to select multiple. At least one is needed for syncing.</p>
       </div>
+      <div class="field" id="c-identities-wrap" hidden>
+        <label>Ticketing account mapping</label>
+        <p class="hint">Username in this project's ticketing system for each assigned consultant. Overrides the consultant's global identity. Leave blank to use their global one.</p>
+        <div id="c-identities"></div>
+      </div>
       <div class="spread">
         <span id="c-test-result" class="hint" aria-live="polite"></span>
         <span>