feat(editor): animate the canvas in step with the simulation

A simulation store owns one run and one playhead, so the chart and the canvas
read off the same instant. While the panel is open, Stocks show a live value and
a fill gauge, Flows and Converters their current value; a play/pause + scrubber
drives the playhead (a wall-clock-paced clock in usePlayback), and a marker
tracks it on the chart.

src/components/Editor.vue

@@ -26,6 +26,7 @@ import {
 } from "@vue-flow/core"
 import { computed, onBeforeUnmount, onMounted, ref, useTemplateRef } from "vue"
 import { useAutosave } from "@/composables/useAutosave"
+import { usePlayback } from "@/composables/usePlayback"
 import { parseModel, serializeModel } from "@/model/io"
 import { project } from "@/model/projection"
 import { type Sample, SAMPLES } from "@/model/samples"
@@ -54,6 +55,10 @@ const edges = computed(() => graph.value.edges)
 // the Model and recomputes reactively while open.
 const showResults = ref(false)
 
+// The playback clock that advances the playhead while a run is playing (mounted
+// once here; the simulation store holds the state it drives).
+usePlayback()
+
 // Explicit shared id: useVueFlow() runs here in the parent setup, before
 // <VueFlow> mounts. Pinning both to the same id guarantees they resolve to one
 // store instance, so the event hooks below actually fire.