map view (Leaflet + OSM), iOS switches, Alarm → Benachrichtigungen

* flats: new lat/lng columns (migration v3); alert geocodes every new flat
  through googlemaps and ships coords in the payload
* web: CSP extended for unpkg (leaflet.css) + tile.openstreetmap.org
* Wohnungen tab: Liste/Karte view toggle (segmented, CSS-only via :has(),
  selection persisted in localStorage). Karte shows passende flats as Pins
  on an OSM tile map; Popup per Pin mit Adresse, Zimmer/m²/€ und Link
* Top-strip toggles are now proper iOS-style toggle switches (single
  rounded knob sliding in a pill, red when on), no descriptive subtitle
* Alarm-Karte verlinkt jetzt auf /einstellungen/benachrichtigungen
  (Filter-Karte bleibt /einstellungen/filter)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

alert/flat.py

@@ -28,6 +28,7 @@ class Flat:
         self.id = self.link  # we could use data.get('id', None) but link is easier to debug
         self.gmaps = maps.Maps()
         self._connectivity = None
+        self._coords = None
         self.address_link_gmaps = f"https://www.google.com/maps/search/?api=1&query={quote(self.address)}"
 
     def __str__(self):
@@ -59,6 +60,12 @@ class Flat:
             self._connectivity = self.gmaps.calculate_score(self.address)
         return self._connectivity
 
+    @property
+    def coords(self):
+        if self._coords is None:
+            self._coords = self.gmaps.geocode(self.address) or (None, None)
+        return self._coords
+
     @property
     def display_address(self):
         if ',' in self.address: