fix: time diff calculation for activities

An error in the logic existed due to a lack of documentation. As a result, the
estimated times were off by up to 30 days.

As an example, an activity that takes place on February 14, 2025 at 17:00 would
state that this was in "1 month and 5 days" (at the time of writing this commit
message). We all know that this is not the case, because that would actually be
January 21, 2025.

The `d` property of a `DateInterval` is now used directly instead of doing our
own calculations with the `days` property (_all_ days between both dates, not
just the difference after substracting years and months).

module/Application/src/View/Helper/TimeDiff.php

@@ -26,21 +26,23 @@ public function __invoke(
         DateTime $end,
     ): string {
         $diffInterval = $start->diff($end);
-        $days = false === $diffInterval->days ? 0 : $diffInterval->days;
-
         $diff = [
             'year' => $diffInterval->y,
             'month' => $diffInterval->m,
-            'week' => (int) floor($days / 7) % 4,
-            'day' => $days % 7,
+            'week' => (int) floor($diffInterval->d / 7),
+            'day' => $diffInterval->d % 7,
             'hour' => $diffInterval->h,
             'minute' => $diffInterval->i,
             'second' => $diffInterval->s,
         ];
 
         $units = [];
         $result = [];
-        if (0 === $days) {
+        if (
+            0 === $diff['year']
+            && 0 === $diff['month']
+            && 0 === $diff['day']
+        ) {
             if (
                 0 === $diff['hour']
                 && 0 === $diff['minute']