fix: use ISO date format for page date storage

Changes dateformat.default from 'd M Y' to 'Y-m-d H:i' so dates
submitted via the post form are stored as '2026-06-19 10:30' rather
than '18 Jun 2026'. This ensures add-page-by-form generates slugs in
YYYY-MM-DD-HHmm-title order, preserving chronological sort.

All templates use explicit |date() filters so display is unaffected.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/system.yaml

@@ -41,7 +41,7 @@ pages:
   list:
     count: 20
   dateformat:
-    default: 'd M Y'
+    default: 'Y-m-d H:i'
     short: 'D, d M Y G:i:s'
     long: 'D, d M Y G:i:s'
   publish_dates: true