refactor(webui): split the sidebar's observability pages out of Workspace (#845)

Workspace was doing two jobs: things you CONFIGURE (Servers, Tools, Secrets,
Agent Tokens) sat in the same list as things you OBSERVE (Sessions, Activity
Log, Security Scanners), so nine items read as one undifferentiated column and
Sessions in particular looked misfiled next to Secrets.

The three observation pages move to their own OBSERVABILITY section, Activity
Log first with Sessions beneath it — they are two lenses on the same data, and
the Sessions page's only action is to open the Activity Log filtered to one
session.

No routes, no components, no behaviour: sidebar grouping only.

Author: Dumbris

frontend/src/components/SidebarNav.vue

@@ -263,27 +263,42 @@
                 <span v-show="!collapsed">Agent Tokens</span>
               </router-link>
             </li>
+          </ul>
+
+          <!-- Section: Observability
+               Workspace is what you CONFIGURE (servers, tools, secrets);
+               these are what you OBSERVE. Keeping them apart stops the two
+               kinds of page reading as one undifferentiated list. -->
+          <div
+            v-if="!collapsed"
+            class="mt-5 mb-1 px-3 text-[10px] font-semibold uppercase tracking-[0.12em] text-base-content/40"
+          >
+            Observability
+          </div>
+          <div v-else class="mt-3 mb-1 mx-auto w-6 h-px bg-base-300"></div>
+
+          <ul class="menu menu-sm w-full gap-0.5 p-0">
             <li>
               <router-link
-                to="/sessions"
-                :class="{ 'active': isActiveRoute('/sessions') }"
+                to="/activity"
+                :class="{ 'active': isActiveRoute('/activity') }"
                 class="rounded-lg font-medium"
-                :title="collapsed ? 'Sessions' : ''"
-                data-test="sidebar-sessions"
+                :title="collapsed ? 'Activity Log' : ''"
               >
-                <IconSessions class="w-5 h-5 shrink-0" />
-                <span v-show="!collapsed">Sessions</span>
+                <IconActivity class="w-5 h-5 shrink-0" />
+                <span v-show="!collapsed">Activity Log</span>
               </router-link>
             </li>
             <li>
               <router-link
-                to="/activity"
-                :class="{ 'active': isActiveRoute('/activity') }"
+                to="/sessions"
+                :class="{ 'active': isActiveRoute('/sessions') }"
                 class="rounded-lg font-medium"
-                :title="collapsed ? 'Activity Log' : ''"
+                :title="collapsed ? 'Sessions' : ''"
+                data-test="sidebar-sessions"
               >
-                <IconActivity class="w-5 h-5 shrink-0" />
-                <span v-show="!collapsed">Activity Log</span>
+                <IconSessions class="w-5 h-5 shrink-0" />
+                <span v-show="!collapsed">Sessions</span>
               </router-link>
             </li>
             <li>