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/zh.json

@@ -1307,6 +1307,8 @@
   "render.image_failed": "渲染失败",
   "render.image_unknown": "渲染状态不可用",
   "render.image_reduce": "请尝试更小的图像或更简单的场景。",
+  "render.image_too_large": "超出图像限制：每边 4096 像素，总计 4,194,304 像素。",
+  "render.image_use_largest": "使用此宽高比下的最大尺寸：",
   "render.image_cancel": "取消渲染",
   "render.image_stale": "此图像使用后续更改之前捕获的场景。",
   "render.image_start": "开始渲染",