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/fr.json

@@ -1170,6 +1170,8 @@
   "render.image_failed": "Échec du rendu",
   "render.image_unknown": "État du rendu indisponible",
   "render.image_reduce": "Essayez une image plus petite ou une scène plus simple.",
+  "render.image_too_large": "Au-delà de la limite d'image : 4096 pixels par côté et 4 194 304 pixels au total.",
+  "render.image_use_largest": "Utiliser la plus grande taille à ce format :",
   "render.image_cancel": "Annuler le rendu",
   "render.image_stale": "Cette image utilise la scène capturée avant les dernières modifications.",
   "render.image_start": "Lancer le rendu",