Add default file blueprint with alt and caption fields

- Create default.yml blueprint for all files
- Add caption (legend) and alt (accessibility) fields
- Update image block blueprint: remove alt/caption fields
- Add help text in image block to remind filling these in image blueprint
- Improve image block UI: cards layout, huge size

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

site/blueprints/files/default.yml

@@ -0,0 +1,11 @@
+title: Fichier
+
+fields:
+  caption:
+    label: Légende
+    type: text
+    help: Légende affichée sous l'image
+  alt:
+    label: Texte alternatif
+    type: text
+    help: Description de l'image pour l'accessibilité et le SEO