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>

frontend/src/features/triage/TaskDetailModal.tsx

@@ -24,6 +24,7 @@ import { UserAvatar } from '../../components/UserAvatar';
 import { UserAutocomplete } from '../UserAutocomplete';
 import { triggerHaptic } from '../../lib/api';
 import { MarkdownViewer } from '../../lib/markdown';
+import { addDays, joinDue, splitDue } from '../../lib/due';
 
 interface TaskDetailModalProps {
   task: Thought | null;
@@ -165,15 +166,15 @@ export const TaskDetailModal: React.FC<TaskDetailModalProps> = ({
     }
   };
 
+  const dueParts = splitDue(task.dueDate);
+
   const handleQuickDueDate = (daysOffset: number | null) => {
     if (daysOffset === null) {
       onUpdateDueDate(task, '');
       return;
     }
-    const d = new Date();
-    d.setDate(d.getDate() + daysOffset);
-    const dateStr = d.toISOString().split('T')[0];
-    onUpdateDueDate(task, dateStr);
+    // Keep a chosen time when only the day changes
+    onUpdateDueDate(task, joinDue(addDays(daysOffset), splitDue(task.dueDate).time));
   };
 
   const currentColumn = activeColumns.find((c) => c.id === task.kanbanStatus) ||
@@ -469,21 +470,32 @@ export const TaskDetailModal: React.FC<TaskDetailModalProps> = ({
             <div className="space-y-1.5">
               <label className="block text-xs font-semibold text-main flex items-center gap-1">
                 <Calendar className="w-3.5 h-3.5 text-accent" />
-                <span>Fälligkeitsdatum:</span>
+                <span>Fällig:</span>
               </label>
               <div className="flex items-center gap-1.5">
                 <input
                   type="date"
-                  value={task.dueDate || ''}
-                  onChange={(e) => onUpdateDueDate(task, e.target.value)}
-                  className="flex-1 h-8 input-field px-2 text-xs bg-panel"
+                  aria-label="Fälligkeitsdatum"
+                  value={dueParts.date}
+                  onChange={(e) => onUpdateDueDate(task, joinDue(e.target.value, dueParts.time))}
+                  className="flex-1 min-w-0 h-11 sm:h-8 input-field px-2 text-base sm:text-xs bg-panel"
+                />
+                <input
+                  type="time"
+                  aria-label="Uhrzeit (optional)"
+                  title="Uhrzeit (optional) – ohne Uhrzeit gilt die Aufgabe den ganzen Tag"
+                  value={dueParts.time}
+                  disabled={!dueParts.date}
+                  onChange={(e) => onUpdateDueDate(task, joinDue(dueParts.date, e.target.value))}
+                  className="w-[7.5rem] shrink-0 h-11 sm:h-8 input-field px-2 text-base sm:text-xs bg-panel disabled:opacity-50"
                 />
                 {task.dueDate && (
                   <button
                     type="button"
                     onClick={() => onUpdateDueDate(task, '')}
-                    className="btn-action h-8 px-2 text-xs text-muted hover:text-status-err"
-                    title="Datum entfernen"
+                    className="btn-action h-11 w-11 sm:h-8 sm:w-8 flex items-center justify-center text-muted hover:text-status-err shrink-0"
+                    title="Fälligkeit entfernen"
+                    aria-label="Fälligkeit entfernen"
                   >
                     <X className="w-3.5 h-3.5" />
                   </button>
@@ -511,6 +523,17 @@ export const TaskDetailModal: React.FC<TaskDetailModalProps> = ({
                 >
                   +1 Woche
                 </button>
+                {dueParts.time && (
+                  <button
+                    type="button"
+                    onClick={() => onUpdateDueDate(task, dueParts.date)}
+                    className="px-1.5 py-0.5 bg-canvas border border-border rounded-btn text-muted hover:text-main inline-flex items-center gap-1"
+                    title="Uhrzeit entfernen – die Aufgabe gilt dann den ganzen Tag"
+                  >
+                    <X className="w-2.5 h-2.5" />
+                    Uhrzeit
+                  </button>
+                )}
               </div>
               {onUpdateRecurrence && (
                 <div className="pt-1">