feat: Phase 4 — Wetter-Widget, Wiederkehrende Aufgaben, Kanban-Ansicht, PWA

- server/routes/weather.js: OpenWeatherMap-Proxy (aktuelles Wetter + 3-Tage-Forecast,
  30-min-Cache, graceful fallback wenn kein API-Key gesetzt)
- public/pages/dashboard.js: Weather-Widget parallel mit Dashboard-Daten laden
- public/styles/dashboard.css: Weather-Widget-Styles (Gradient, Forecast-Strip)
- server/services/recurrence.js: RRULE-Parser (FREQ=DAILY/WEEKLY/MONTHLY, BYDAY,
  INTERVAL, UNTIL) + nextOccurrence()-Funktion
- server/routes/tasks.js: Bei PATCH /:id/status = done → nächste Instanz
  wiederkehrender Aufgaben automatisch anlegen
- public/pages/tasks.js: Kanban-Ansicht (3 Spalten: Offen/In Bearbeitung/Erledigt)
  mit HTML5 Drag & Drop, View-Toggle (Liste/Kanban)
- public/styles/tasks.css: Kanban-Board-Styles (Spalten, Cards, Drag-over-Highlight)
- public/sw.js: Cache-Version auf v2, alle Modul-CSS-Dateien im APP_SHELL-Cache

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/styles/tasks.css

@@ -495,6 +495,128 @@
   margin-left: auto;
 }
 
+/* --------------------------------------------------------
+ * Kanban-Board
+ * -------------------------------------------------------- */
+.kanban-board {
+  display: grid;
+  grid-template-columns: repeat(3, 1fr);
+  gap: var(--space-3);
+  align-items: start;
+  min-height: 60vh;
+}
+
+@media (max-width: 640px) {
+  .kanban-board {
+    grid-template-columns: 1fr;
+  }
+}
+
+.kanban-col {
+  background-color: var(--color-surface-2);
+  border-radius: var(--radius-md);
+  display: flex;
+  flex-direction: column;
+  min-height: 200px;
+}
+
+.kanban-col__header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: var(--space-3) var(--space-4);
+  border-bottom: 1.5px solid var(--color-border);
+}
+
+.kanban-col__title {
+  font-size: var(--text-sm);
+  font-weight: var(--font-weight-semibold);
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+}
+
+.kanban-col__count {
+  font-size: var(--text-xs);
+  color: var(--color-text-disabled);
+  background-color: var(--color-surface);
+  padding: 2px var(--space-2);
+  border-radius: var(--radius-full);
+}
+
+.kanban-col__body {
+  flex: 1;
+  padding: var(--space-3);
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-2);
+  min-height: 80px;
+  transition: background-color var(--transition-fast);
+}
+
+.kanban-col__body--over {
+  background-color: var(--color-accent-light);
+  border-radius: 0 0 var(--radius-md) var(--radius-md);
+}
+
+.kanban-card {
+  background-color: var(--color-surface);
+  border-radius: var(--radius-sm);
+  box-shadow: var(--shadow-sm);
+  padding: var(--space-3);
+  cursor: grab;
+  user-select: none;
+  transition: box-shadow var(--transition-fast), opacity var(--transition-fast),
+              transform var(--transition-fast);
+}
+
+.kanban-card:hover {
+  box-shadow: var(--shadow-md);
+}
+
+.kanban-card--dragging {
+  opacity: 0.4;
+  cursor: grabbing;
+  transform: rotate(1.5deg);
+}
+
+.kanban-card--done {
+  opacity: 0.6;
+}
+
+.kanban-card__title {
+  font-size: var(--text-sm);
+  font-weight: var(--font-weight-medium);
+  color: var(--color-text-primary);
+  margin-bottom: var(--space-2);
+  line-height: 1.4;
+}
+
+.kanban-card--done .kanban-card__title {
+  text-decoration: line-through;
+  color: var(--color-text-secondary);
+}
+
+.kanban-card__meta {
+  display: flex;
+  flex-wrap: wrap;
+  gap: var(--space-2);
+  margin-bottom: var(--space-2);
+}
+
+.kanban-card__footer {
+  display: flex;
+  justify-content: flex-end;
+  margin-top: var(--space-2);
+}
+
+.kanban-drop-placeholder {
+  height: 60px;
+  border: 2px dashed var(--color-accent);
+  border-radius: var(--radius-sm);
+  background-color: var(--color-accent-light);
+  opacity: 0.5;
+}
+
 /* --------------------------------------------------------
  * Leer-Zustand
  * -------------------------------------------------------- */