Render preview: draw within the pixel budget on 4K/5K screens (#1464) (#1477)

The product preview asked the viewport executor for the full physical
viewport, so a maximised 4K window (~8 MP) was refused against the 4 MP
limit on every frame. The preview now draws at the largest size of the
same aspect the executor admits (RenderLimits::fit, scratch included)
and filters it up into the viewport through the existing offscreen
blit. Viewports within the limits still draw at full size.

PNG setups past the image limits now say what the limits are and the
largest size at that aspect, and the image panel offers to use it.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

assets/locales/ko.json

@@ -1049,6 +1049,8 @@
   "render.image_failed": "렌더링 실패",
   "render.image_unknown": "렌더 상태를 확인할 수 없습니다",
   "render.image_reduce": "더 작은 이미지나 더 단순한 장면을 사용해 보세요.",
+  "render.image_too_large": "이미지 한도를 초과했습니다: 한 변 4096픽셀, 전체 4,194,304픽셀.",
+  "render.image_use_largest": "이 비율에서 가장 큰 크기 사용:",
   "render.image_cancel": "렌더링 취소",
   "render.image_stale": "이 이미지는 후속 변경 전에 캡처한 장면을 사용합니다.",
   "render.image_start": "렌더링 시작",