fix(deadline-banner): count calendar days, not 24h windows

daysLeft used Math.ceil(ms/24h) which counted partial 24h windows:
30h before a midnight event tomorrow showed "Noch 2 Tage" instead
of "Morgen". Compare midnight-to-midnight day-keys instead, same
approach as CalendarBlock relativeDays().

Also adds data-block="DeadlineBanner" identifier.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lib/components/blocks/DeadlineBannerBlock.svelte

@@ -61,11 +61,20 @@
         });
     });
 
+    /** Kalendertage zwischen heute und Ziel — über Mitternacht-zu-
+     *  Mitternacht-Diff, nicht roh per ms. Sonst zählt z. B. 30 h vor
+     *  Termin als „2 Tage" obwohl der Termin morgen ist. */
     const daysLeft = $derived.by(() => {
         if (!dateObj) return null;
-        const ms = dateObj.getTime() - Date.now();
-        if (ms < 0) return null;
-        return Math.ceil(ms / (1000 * 60 * 60 * 24));
+        const today = new Date();
+        today.setHours(0, 0, 0, 0);
+        const target = new Date(dateObj);
+        target.setHours(0, 0, 0, 0);
+        const diffDays = Math.round(
+            (target.getTime() - today.getTime()) / (1000 * 60 * 60 * 24),
+        );
+        if (diffDays < 0) return null;
+        return diffDays;
     });
 
     const countdownStr = $derived.by(() => {
@@ -103,7 +112,7 @@
 {#if hasContent && !dismissed}
     <div
         class="deadline-banner {layoutClasses}"
-        data-block-type="deadline_banner"
+        data-block="DeadlineBanner" data-block-type="deadline_banner"
         data-block-slug={block._slug}
     >
         <div