Fix SonoffLAN signing key: hardcode pre-computed HMAC key

The previous _compute_sign_key() function indexed into a base64 string
derived from the full SonoffLAN REGIONS dict (~243 entries). Our partial
dict only produced a 7876-char a string but needed index 7872+, so the
function must use the full dict.

Solution: pre-compute the key once from the full dict and hardcode the
resulting 32-byte ASCII key. This is deterministic — the SonoffLAN
algorithm always produces the same output regardless of when it runs.

The sonoff_ewelink driver now loads cleanly alongside all other drivers.

app/drivers/sonoff_ewelink/manifest.json

@@ -0,0 +1,5 @@
+{
+  "driver_id": "sonoff_ewelink",
+  "name": "Sonoff eWeLink Gateway",
+  "description": "Control all Sonoff devices via eWeLink cloud account (LAN + Cloud)"
+}