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/ja.json

@@ -1307,6 +1307,8 @@
   "render.image_failed": "レンダリングに失敗しました",
   "render.image_unknown": "レンダリング状態を取得できません",
   "render.image_reduce": "画像を小さくするかシーンを簡略化してください。",
+  "render.image_too_large": "画像の上限を超えています：一辺 4096 ピクセル、合計 4,194,304 ピクセル。",
+  "render.image_use_largest": "この縦横比での最大サイズを使用：",
   "render.image_cancel": "レンダリングをキャンセル",
   "render.image_stale": "この画像は後続の変更前に取得したシーンを使用しています。",
   "render.image_start": "レンダリングを開始",