feat(ui): toggle rendered frame rate with backtick

src/ui/styles/foundation.css

@@ -591,6 +591,15 @@ body.cockpit-mode #cockpit-cloud-effects.active {
   text-transform: uppercase;
 }
 
+#title-bar .frame-rate-readout {
+  margin-top: 6px;
+  font-family: var(--font-mono);
+  font-size: 11px;
+  font-variant-numeric: tabular-nums;
+  letter-spacing: 1px;
+  color: var(--accent);
+}
+
 /* ── Style Indicator ──────────────────────────── */
 #style-indicator {
   position: fixed;

src/ui/styles/responsive.css

@@ -41,6 +41,19 @@
     width: 36px;
   }
 
+  #title-bar .frame-rate-readout {
+    position: fixed;
+    top: 52px;
+    left: 20px;
+    margin: 0;
+    font-size: 10px;
+    line-height: 12px;
+  }
+
+  #title-bar:has(.frame-rate-readout:not([hidden])) .subtitle {
+    visibility: hidden;
+  }
+
   #location-bar {
     left: 16px;
     right: 16px;