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/static/js/profile.js

@@ -56,12 +56,16 @@ form.addEventListener('submit', async (e) => {
         links,
       },
       extra,
-      settings: { theme: document.getElementById('p-theme').value },
+      settings: {
+        theme: document.getElementById('p-theme').value,
+        navLayout: document.getElementById('p-nav').value,
+      },
     });
     version = res.user.version;
     const theme = res.user.settings.theme;
     document.documentElement.dataset.theme = theme;
     try { localStorage.setItem('theme', theme); } catch (err) { /* ignore */ }
+    document.body.dataset.nav = res.user.settings.navLayout || 'side';
     okBox.textContent = 'Profile saved.';
   } catch (err) {
     if (err.status === 409) {