feat: Ein-/Auswinterung und Winterverluste je Volk

- WinterCheck per colony, winter and season (autumn/spring), every field
  editable; loss causes limited to the COLOSS monitoring categories.
- Summary per winter and apiary: loss rate lost/wintered, marked
  provisional while colonies are not yet checked in spring - an unchecked
  colony never counts as survived.
- A loss marks the colony deceased only on explicit request, in the same
  transaction and audited.
- Permissions winter-checks:read/create/update granted by migration to
  beekeeper and staff roles; winter checks included in backups.
- Page "Überwinterung": next action first, one button per row until the
  colony is wintered, German number format.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

ApiCore-Frontend/src/app/(app)/sidebar.tsx

@@ -20,6 +20,7 @@ import {
   Package,
   Scissors,
   Search,
+  Snowflake,
   ShoppingBag,
   Users,
   Warehouse,
@@ -58,6 +59,12 @@ const NAV_GROUPS: NavGroup[] = [
       { label: "Bienenstände", href: "/apiaries", icon: MapPin },
       { label: "Völker", href: "/colonies", icon: Layers },
       { label: "Königinnen", href: "/queens", icon: Crown },
+      {
+        label: "Überwinterung",
+        href: "/winter",
+        icon: Snowflake,
+        permission: "winter-checks:read",
+      },
     ],
   },
   {