feat: Change QR payment message separator from "/" to ":"

Format is now "Name: MM/YYYY+MM/YYYY" instead of "Name / MM/YYYY+MM/YYYY"
for clearer readability when multiple months are included.

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

templates/juniors.html

@@ -848,7 +848,7 @@
             const numericMonth = rawMonth.includes('+')
                 ? rawMonth.split('+').map(p => p.replace(/(\d{4})-(\d{2})/, '$2/$1')).join('+')
                 : rawMonth.replace(/(\d{4})-(\d{2})/, '$2/$1');
-            const message = `${name} / ${numericMonth}`;
+            const message = `${name}: ${numericMonth}`;
             const qrTitle = document.getElementById('qrTitle');
             const qrImg = document.getElementById('qrImg');
             const qrAccount = document.getElementById('qrAccount');