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/TriageDeck.tsx

@@ -20,6 +20,7 @@ import {
   MoreHorizontal,
 } from 'lucide-react';
 import { SharedList, Thought } from '../../lib/types';
+import { addDays, formatDue } from '../../lib/due';
 import { api, triggerHaptic } from '../../lib/api';
 
 interface TriageDeckProps {
@@ -223,10 +224,7 @@ export const TriageDeck: React.FC<TriageDeckProps> = ({
   const [cards, setCards] = useState<Thought[]>(deck);
   const [lastTriaged, setLastTriaged] = useState<Thought | null>(null);
   const [showDateModal, setShowDateModal] = useState(false);
-  const [taskDueDate, setTaskDueDate] = useState(() => {
-    const tomorrow = new Date(Date.now() + 86400000);
-    return tomorrow.toISOString().slice(0, 10);
-  });
+  const [taskDueDate, setTaskDueDate] = useState(() => addDays(1));
   const [showListSelector, setShowListSelector] = useState(false);
 
   // Long-Press Action Sheet state (for Triage Card or Queue Item)
@@ -666,7 +664,7 @@ export const TriageDeck: React.FC<TriageDeckProps> = ({
 
               {activeCard.dueDate && (
                 <span className="px-2 py-0.5 bg-canvas text-muted border border-border rounded-btn text-xs">
-                  📅 {activeCard.dueDate}
+                  📅 {formatDue(activeCard.dueDate)}
                 </span>
               )}
             </div>