Fix hydration mismatch in CallPanel that crashed room switching

The SSR branch rendered an empty call-panel div while the hydrate
branch expected a child button node. On any full load/refresh of the
lobby room, the mismatched hydration cursor hit tachys's
unreachable!() panic path, trapping the wasm instance and killing all
reactivity (routing, room switching, SSE) for the rest of the page
load, while the already-rendered SSR HTML stayed visually intact.

SSR now renders the same default join-button markup hydrate expects.

src/app.rs

@@ -495,10 +495,22 @@ fn CallPanel(room: Memo<String>, me: String) -> impl IntoView {
         }
             .into_any()
     }
+    // Must mirror the hydrate branch's default (not-in-call) markup exactly -
+    // hydration reconciles this SSR output against what the hydrate branch
+    // above expects to find, and an empty div here (vs. the button hydrate
+    // wants) is a hydration mismatch that panics and traps the whole wasm
+    // instance, killing all reactivity on the page.
     #[cfg(not(feature = "hydrate"))]
     {
         let _ = (room, me);
-        view! { <div class="call-panel"></div> }.into_any()
+        view! {
+            <div class="call-panel">
+                <button class="call-join" disabled=true>
+                    "☎ join call"
+                </button>
+            </div>
+        }
+            .into_any()
     }
 }