feat: add ?scroll query param to set scroll sensitivity on load (#681)

Follows the same pattern as the existing ?volume param. Lets users
bake a preferred scroll sensitivity into the URL (useful for cast/embed
setups where the settings panel isn't reachable), and it's persisted
to localStorage afterwards so it sticks on future visits.

Fixes #607

client/src/app.vue

@@ -206,6 +206,11 @@
       return Math.max(0.0, Math.min(!isNaN(numberParam) ? numberParam * 100 : 100, 100))
     }
 
+    get scroll() {
+      const numberParam = parseInt(new URL(location.href).searchParams.get('scroll') || '', 10)
+      return Math.max(1, Math.min(!isNaN(numberParam) ? numberParam : 10, 100))
+    }
+
     get isCastMode() {
       return !!new URL(location.href).searchParams.get('cast')
     }
@@ -229,6 +234,13 @@
       }
     }
 
+    @Watch('scroll', { immediate: true })
+    onScroll(scroll: number) {
+      if (new URL(location.href).searchParams.has('scroll')) {
+        this.$accessor.settings.setScroll(scroll)
+      }
+    }
+
     @Watch('hideControls', { immediate: true })
     onHideControls(enabled: boolean) {
       if (enabled) {