✨ Add Penpot-specific board size presets (#11565)

Add a "PENPOT" section to the board size presets list with sizes for
File thumbnail, Template cover, Plugin icon and Plugin cover, so
designers no longer need to look up these dimensions manually when
creating thumbnails for files, template submissions or plugin
listings.

Closes #11561

AI-assisted-by: claude-sonnet-5

Signed-off-by: Andres Gonzalez <andres.gonzalez79@gmail.com>

frontend/src/app/main/constants.cljs

@@ -299,7 +299,21 @@
     :height 1152}
    {:name "YouTube thumb"
     :width 1280
-    :height 720}])
+    :height 720}
+
+   {:name "PENPOT"}
+   {:name "File thumbnail"
+    :width 300
+    :height 200}
+   {:name "Template cover"
+    :width 1390
+    :height 781}
+   {:name "Plugin icon"
+    :width 400
+    :height 400}
+   {:name "Plugin cover"
+    :width 1390
+    :height 724}])
 
 (def max-input-length 255)