fix(studio): centre Welcome page content on wide windows

Welcome page used ConstrainedBox(maxWidth: 960) inside a
left-aligned Column, so on the 1440x900 default window
the content hugged the left edge and left ~370 px of
empty space on the right. With Stefan's M4 sidebar
collapsed to 72 px the asymmetry was the dominant visual
in the first thing every operator sees.

Wrap the ConstrainedBox in a Center widget. 960 px stays
an opinionated reading width — long-form prose gets
uncomfortable past ~1100 px — but the slack now splits
evenly so the content sits with ~190 px breathing room
on both sides instead of all on one.

Bumps the editor-package lock pointer to ad2e5b5 (same
0.2.2 version, picks up the extract-with-approval test
commit on top of f8138f3) as a side-effect of the
flutter pub upgrade run during verification.

Version 0.52.2 -> 0.52.3.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

pubspec.lock

@@ -125,7 +125,7 @@ packages:
     description:
       path: "."
       ref: main
-      resolved-ref: f8138f3516098dc03e65fdfa52916885e7e7aeff
+      resolved-ref: ad2e5b50d5201c9b97329b02e2223d6e599545f1
       url: "https://git.flemming.ai/fai/studio-flow-editor"
     source: git
     version: "0.2.2"