Add Hot Spring Connected Spa integration

Unofficial cloud integration for Watkins/Hot Spring "Connected Spa" hot tubs.
Reverse-engineered from the official Android app.

- climate entity (setpoint + current water temp) via heater/control
- sensors: water temperature, salt output level, salt cartridge
- REST (JWT) login + spa discovery; mTLS fetch of rotating HiveMQ broker creds;
  MQTT subscribe (telemetry) / publish (control)
- config flow prompts for account email + password (stored encrypted, never in repo)
- no secrets committed: vendor mTLS client cert is loaded from host PEM files
  (see README); .gitignore blocks pem/p12/der/key/env

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

@@ -0,0 +1,31 @@
+{
+  "config": {
+    "step": {
+      "user": {
+        "title": "Hot Spring Connected Spa",
+        "description": "Sign in with your Hot Spring Spas app account. The mutual-TLS client certificate must already be present on the host (see the integration README).",
+        "data": {
+          "email": "E-mail",
+          "password": "Password",
+          "cert_dir": "Certificate directory"
+        }
+      }
+    },
+    "error": {
+      "invalid_auth": "Invalid e-mail or password.",
+      "cannot_connect": "Could not reach the Hot Spring cloud.",
+      "missing_certs": "The client certificate files were not found in the certificate directory.",
+      "unknown": "Unexpected error."
+    },
+    "abort": {
+      "already_configured": "This spa account is already configured."
+    }
+  },
+  "entity": {
+    "sensor": {
+      "water_temperature": { "name": "Water temperature" },
+      "salt_output_level": { "name": "Salt output level" },
+      "cartridge": { "name": "Salt cartridge" }
+    }
+  }
+}