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/export_markdown.go

@@ -141,7 +141,8 @@ func taskMarkdownLine(t domain.Thought) string {
 		line += " 🔁 every " + every
 	}
 	if t.DueDate != "" {
-		line += " 📅 " + t.DueDate
+		// Obsidian Tasks only knows the date; the time follows separated by a space
+		line += " 📅 " + strings.Replace(t.DueDate, "T", " ", 1)
 	}
 	var sb strings.Builder
 	sb.WriteString(line + "\n")