Add AI weekly and monthly summaries with archive UI and backup support

src/helpers.php

@@ -197,6 +197,87 @@ function format_display_date(string $date, bool $withWeekday = true): string
     return $weekdays[(int) $current->format('w')] . ', ' . $label;
 }
 
+function format_display_datetime(string $value): string
+{
+    try {
+        $current = new DateTimeImmutable($value);
+    } catch (Throwable) {
+        return $value;
+    }
+
+    $months = [
+        1 => 'Januar',
+        2 => 'Februar',
+        3 => 'März',
+        4 => 'April',
+        5 => 'Mai',
+        6 => 'Juni',
+        7 => 'Juli',
+        8 => 'August',
+        9 => 'September',
+        10 => 'Oktober',
+        11 => 'November',
+        12 => 'Dezember',
+    ];
+
+    return $current->format('j.') . ' ' . $months[(int) $current->format('n')] . ' ' . $current->format('Y') . ' um ' . $current->format('H:i');
+}
+
+function iso_week_key(string $date): string
+{
+    $current = DateTimeImmutable::createFromFormat('Y-m-d', $date);
+
+    if ($current === false) {
+        return date('o-\K\W-W');
+    }
+
+    return $current->format('o-\K\W-W');
+}
+
+function month_key(string $date): string
+{
+    $current = DateTimeImmutable::createFromFormat('Y-m-d', $date);
+
+    if ($current === false) {
+        return date('Y-m');
+    }
+
+    return $current->format('Y-m');
+}
+
+function iso_week_label(string $key): string
+{
+    if (preg_match('/^(\d{4})-KW-(\d{2})$/', $key, $matches) === 1) {
+        return 'KW ' . $matches[2] . ' / ' . $matches[1];
+    }
+
+    return $key;
+}
+
+function month_label(string $key): string
+{
+    if (preg_match('/^(\d{4})-(\d{2})$/', $key, $matches) !== 1) {
+        return $key;
+    }
+
+    $months = [
+        '01' => 'Januar',
+        '02' => 'Februar',
+        '03' => 'März',
+        '04' => 'April',
+        '05' => 'Mai',
+        '06' => 'Juni',
+        '07' => 'Juli',
+        '08' => 'August',
+        '09' => 'September',
+        '10' => 'Oktober',
+        '11' => 'November',
+        '12' => 'Dezember',
+    ];
+
+    return ($months[$matches[2]] ?? $matches[2]) . ' ' . $matches[1];
+}
+
 function icon_path(string $name): string
 {
     return '/assets/icons/' . $name . '.svg';