fix: shorten cancellation notification label to prevent wrapping on mobile

[harshitbshah]

Problem

In the subscription edit form, the Notify me dropdown and the Cancellation Notification date picker are laid out side by side in a 66/33 split (desktop) or 50/50 split (mobile). On mobile, the label "Cancellation Notification" is too long to fit on a single line at 50% column width, causing it to wrap to two lines. This pushes the date input down, misaligning it with the "Notify me" dropdown next to it — making the form look broken and harder to use.

Before:

Notify me          Cancellation
[3 days before ▼]  Notification
                   [date input  ]

After:

Notify me          Cancel reminder
[3 days before ▼]  [date input     ]

Fix

Shortened the English label for the cancellation_notification key from "Cancellation Notification" to "Cancel reminder" in includes/i18n/en.php.

• "Cancel reminder" is 15 characters vs 24 for "Cancellation Notification" — fits comfortably on one line even on the smallest common mobile viewport (320px, ~160px per column)
• The meaning is preserved: it's a date field that triggers a reminder to cancel the subscription
• Only the English label is changed — other language translations are unaffected and remain the responsibility of their respective contributors

Why not a CSS fix?

An alternative approach would be to stack the two fields vertically on mobile (flex-wrap: wrap). However, this is a more invasive layout change and unnecessary — the label text is the root cause. A shorter label is simpler, less risky, and keeps the compact side-by-side layout that works well when labels fit on one line.