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/dashboard.css

@@ -329,6 +329,95 @@
   overflow: hidden;
 }
 
+/* --------------------------------------------------------
+ * Wetter-Widget
+ * -------------------------------------------------------- */
+.weather-widget {
+  background: linear-gradient(135deg, #1a73e8 0%, #0f4fa8 100%);
+  color: #ffffff;
+}
+
+.weather-widget__main {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: var(--space-4) var(--space-5);
+}
+
+.weather-widget__temp {
+  font-size: var(--text-4xl, 2.25rem);
+  font-weight: var(--font-weight-bold);
+  line-height: 1;
+  margin-bottom: var(--space-1);
+}
+
+.weather-widget__desc {
+  font-size: var(--text-base);
+  font-weight: var(--font-weight-medium);
+  text-transform: capitalize;
+  margin-bottom: 2px;
+}
+
+.weather-widget__city {
+  font-size: var(--text-sm);
+  opacity: 0.85;
+  margin-bottom: var(--space-2);
+}
+
+.weather-widget__meta {
+  font-size: var(--text-xs);
+  opacity: 0.75;
+  line-height: 1.4;
+}
+
+.weather-widget__icon {
+  width: 80px;
+  height: 80px;
+  flex-shrink: 0;
+  filter: drop-shadow(0 2px 4px rgba(0,0,0,0.2));
+}
+
+.weather-forecast {
+  display: flex;
+  border-top: 1px solid rgba(255,255,255,0.2);
+  padding: var(--space-3) var(--space-5);
+  gap: var(--space-4);
+}
+
+.weather-forecast__day {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 2px;
+  flex: 1;
+}
+
+.weather-forecast__label {
+  font-size: var(--text-xs);
+  font-weight: var(--font-weight-semibold);
+  opacity: 0.85;
+  text-transform: capitalize;
+}
+
+.weather-forecast__icon {
+  width: 32px;
+  height: 32px;
+}
+
+.weather-forecast__temps {
+  display: flex;
+  gap: var(--space-2);
+  font-size: var(--text-xs);
+}
+
+.weather-forecast__high {
+  font-weight: var(--font-weight-semibold);
+}
+
+.weather-forecast__low {
+  opacity: 0.65;
+}
+
 /* --------------------------------------------------------
  * Skeleton-Zustände (pro Widget)
  * -------------------------------------------------------- */