feat(ui): render the FPS readout in the title bar from the status store

createFrameRateMonitor() keeps the key, frame count and interval and
publishes the readout to the gev-status store; the scene chrome renders
it after the subtitle with the same class, hidden state and title. A
page without the title bar still gets no monitor, and a monitor only
updates or withdraws the readout the store shows.

docs/VUE-MIGRATION.md

@@ -149,7 +149,8 @@ a feature supplying only its accent.
    `gev-status` store; `SplitFlapText` plays the departure-board flip that
    `splitFlap.js` plans, writing only the label's one text node; the toast
    renders what each owner's `createToast()` publishes there, on that
-   owner's own timer), the Scene actions panel (`SceneInteractions`
+   owner's own timer, and the title bar's FPS readout what
+   `createFrameRateMonitor()` publishes), the Scene actions panel (`SceneInteractions`
    teleports what the scene interactions owner publishes to the
    `gev-scene-interactions` store into the viewer's container; the owner
    keeps the pick handler and focuses the picked action), the data-pack