fix(capture): preserve PhotoStage preview aspect on first paint

Contain-fit waits for a real parent width, uses ResizeObserver, and shrinks
width when maxH would otherwise stretch the CSS box. Pure fitDisplay is
covered by node tests.

frontend/src/capture/fitDisplay.ts

@@ -0,0 +1,17 @@
+// Contain-fit of an image into a parent width with a height cap. Pure and
+// display-only; all measurement math stays on source pixels.
+// Null when the parent is not laid out yet (clientWidth 0) — the caller must
+// not finalize CSS size until it is, to avoid a wrong-aspect flash.
+export function fitDisplay(
+  parentW: number, imgW: number, imgH: number, maxH: number,
+): { fitW: number; fitH: number } | null {
+  if (parentW <= 0 || imgW <= 0 || imgH <= 0) return null;
+  const fitW = parentW;
+  let fitH = (imgH / imgW) * fitW;
+  if (fitH > maxH) {
+    // Cap height and shrink width to preserve aspect; never stretch.
+    fitH = maxH;
+    return { fitW: (fitH * imgW) / imgH, fitH };
+  }
+  return { fitW, fitH };
+}