Add lights/jets/salt control, app-traffic mimicry, and optional local read path

- New entities: number (salt output level 0-10), switch (lights, jets); the
  read-only salt sensor is replaced by the settable number.
- Look identical to the official app on the wire: send the app's Dalvik UA on
  REST, okhttp/4.12.0 on the mTLS creds fetch, and "mqtt-<uuid>" client ids.
- Consolidate every constant value into constants.py (well commented); protocol.py
  keeps only the command builders; const.py removed.
- Optional local read path: a `local_status_url` config option polls the dongle's
  local /status endpoint for telemetry instead of the cloud MQTT subscription
  (control stays cloud MQTT). State getters handle both schemas.

No private/internal values in the repo (the spa IP is runtime config only).

Fixes bug-41mqxddz7zeh

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F6B8b3iYNv6QUftK2FDfYb

custom_components/hotspring/strings.json

@@ -7,7 +7,11 @@
         "data": {
           "email": "E-mail",
           "password": "Password",
-          "cert_dir": "Certificate directory"
+          "cert_dir": "Certificate directory",
+          "local_status_url": "Local /status URL (optional)"
+        },
+        "data_description": {
+          "local_status_url": "If Home Assistant is on the spa's LAN, e.g. http://<spa-ip>/status, monitoring is polled locally instead of over the cloud; control still uses the cloud."
         }
       }
     },
@@ -24,8 +28,14 @@
   "entity": {
     "sensor": {
       "water_temperature": { "name": "Water temperature" },
-      "salt_output_level": { "name": "Salt output level" },
       "cartridge": { "name": "Salt cartridge" }
+    },
+    "number": {
+      "salt_output_level": { "name": "Salt output level" }
+    },
+    "switch": {
+      "lights": { "name": "Lights" },
+      "jets": { "name": "Jets" }
     }
   }
 }