feat(tasks): due dates with optional time; alarms at the right moment

Due dates can carry a time (YYYY-MM-DDTHH:MM). Alarms fire at that time,
or 09:00 for all-day tasks, minus the user's lead time, in TZ (default
Europe/Berlin, tzdata embedded). A new due date or reopening a task
re-arms the alarm. Recurrence keeps the time; Markdown export/import
carry it. The UI shows relative labels ("Heute, 14:30"), marks overdue
tasks to the minute and groups by local day instead of the UTC date.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/import_markdown.go

@@ -759,7 +759,7 @@ func MarkdownToBlocks(md string) []domain.DocumentBlock {
 
 var (
 	recurRe    = regexp.MustCompile(`(?i)[ \t]*🔁[ \t]*every[ \t]+(day|week|month|year)\b`)
-	dueRe      = regexp.MustCompile(`[ \t]*📅[ \t]*(\d{4}-\d{2}-\d{2})`)
+	dueRe      = regexp.MustCompile(`[ \t]*📅[ \t]*(\d{4}-\d{2}-\d{2})(?:[ T](\d{2}:\d{2}))?`)
 	priorityRe = regexp.MustCompile(`[ \t]*(🔺|⏫|🔼|🔽|⏬)`)
 )
 
@@ -801,6 +801,9 @@ func parseTaskFile(body string) []importTask {
 		}
 		if d := dueRe.FindStringSubmatch(t.content); d != nil {
 			t.due = d[1]
+			if d[2] != "" {
+				t.due += "T" + d[2]
+			}
 			t.content = dueRe.ReplaceAllString(t.content, "")
 		}
 		if p := priorityRe.FindStringSubmatch(t.content); p != nil {