fix: prevent phosphor sweep speed burst on initial load

Cap the phosphor accumulator so excess time from dropped frames is
discarded rather than draining over subsequent frames at max tick rate.
Also reset timing state when the animation loop starts.

Author: sandrwich

src/ui/RotatorWindow.svelte

@@ -390,10 +390,8 @@
       const pc = phosphorCtx;
 
       // Run phosphor simulation at fixed 60Hz — accumulate real time, consume in ticks
-      phosphorAccum += dt;
-      const maxTicks = PHOSPHOR_MAX_TICKS;
+      phosphorAccum = Math.min(phosphorAccum + dt, PHOSPHOR_MAX_TICKS * PHOSPHOR_TICK);
       let ticks = Math.floor(phosphorAccum / PHOSPHOR_TICK);
-      if (ticks > maxTicks) ticks = maxTicks;
       phosphorAccum -= ticks * PHOSPHOR_TICK;
 
       for (let t = 0; t < ticks; t++) {
@@ -775,6 +773,8 @@
   $effect(() => {
     if (canvasEl) {
       initCanvas();
+      lastFrameTime = 0;
+      phosphorAccum = 0;
       canvasEl.addEventListener('wheel', onCanvasWheel, { passive: false });
       animFrameId = requestAnimationFrame(drawFrame);
       return () => {