fix(ha-integration): ausgeschaltete Lampe -> "unavailable" statt Fehlerzustand

War eine Lampe stromlos, wurde der Verbindungsfehler als UpdateFailed
gemeldet; HA schob den Config-Entry in SETUP_RETRY und die Integration
fiel in den Fehlerzustand. Jetzt wird Connection-Refused/Timeout als
NinlightUnavailable gefuehrt: der Coordinator liefert den letzten
bekannten State mit available=False, der Entry bleibt geladen und die
Entity zeigt "unavailable". Protokollfehler (400, kaputtes JSON) und
Auth (401 -> Reauth) verhalten sich unveraendert.

Light-Faehigkeiten (RGB/CT, Kelvin-Bereich) sind deshalb dynamische
Properties — ein Entry, der mit stromloser Lampe angelegt wurde,
erkennt sie, sobald die Lampe antwortet.

ha-integration/custom_components/ninlight/client.py

@@ -8,7 +8,7 @@ client must not keep a hot connection pool open.
 
 from __future__ import annotations
 
-from dataclasses import dataclass, field
+from dataclasses import dataclass, field, replace as _dc_replace
 from typing import Any
 
 import httpx
@@ -24,6 +24,14 @@ class NinlightAuthError(NinlightError):
     """The bulb rejected our token (HTTP 401)."""
 
 
+class NinlightUnavailable(NinlightError):
+    """The bulb could not be reached at all (connection refused or timeout).
+
+    A bulb that is switched off or has lost Wi-Fi is an expected state, not a fault,
+    so the coordinator treats this differently from protocol errors.
+    """
+
+
 class NinlightBadRequest(NinlightError):
     """The bulb rejected the payload (HTTP 400) with the reason in its body."""
 
@@ -69,6 +77,19 @@ class NinlightState:
     # device_types are board ids (nl_boards[] in ninlight-w600: "evk", ...), so the
     # name is not a signal, but the keys are.
     strip: bool = False
+    # False when the last poll could not reach the bulb (powered off / offline).
+    # Kept separate from `on` so the entity can show "unavailable" instead of an
+    # error, while the light keeps its last known appearance.
+    available: bool = True
+
+    @classmethod
+    def unavailable_copy(cls, last: "NinlightState") -> "NinlightState":
+        """The last known state, flagged unavailable.
+
+        Keeps the light's appearance (on/off, color) at its last real value while the
+        entity shows as unavailable, instead of falling back to defaults.
+        """
+        return _dc_replace(last, available=False)
 
     @classmethod
     def from_api(
@@ -149,6 +170,9 @@ class NinlightClient:
             resp = await self._session.request(method, f"{self._base}{path}", json=payload)
         except httpx.InvalidURL as err:  # programming error, not a transient one
             raise NinlightError(f"invalid bulb URL: {err}") from err
+        except (httpx.ConnectError, httpx.ConnectTimeout, httpx.ReadTimeout) as err:
+            # No answer at all: bulb powered off, rebooting, or gone from the WLAN.
+            raise NinlightUnavailable(f"bulb unreachable: {err}") from err
         except httpx.HTTPError as err:
             raise NinlightError(f"request to bulb failed: {err}") from err