fix: keep terminal route mounted across page switches

Keep the Terminal component mounted and toggle visibility via CSS so tab state and WebSocket sessions persist when navigating between sidebar pages, while preserving existing per-tab close and full teardown cleanup behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dashboard/frontend/src/App.svelte

@@ -146,8 +146,6 @@
           <Gitea />
         {:else if page === "files" && hasPageAccess("files")}
           <Files />
-        {:else if page === "terminal" && hasPageAccess("terminal")}
-          <Terminal />
         {:else if page === "openclaw" && hasPageAccess("openclaw")}
           <OpenClaw />
         {:else if page === "chat-digest" && hasPageAccess("chat-digest")}
@@ -162,9 +160,15 @@
           <CcConnect />
         {:else if page === "info-engine" && hasPageAccess("dashboard")}
           <InfoEngine />
-        {:else}
+        {:else if page !== "terminal"}
           <Dashboard />
         {/if}
+
+        {#if hasPageAccess("terminal")}
+          <div class:hidden={page !== "terminal"}>
+            <Terminal />
+          </div>
+        {/if}
       </div>
     </main>
   </div>