feat(api): explicit enable toggle for WiFi password endpoint

Previously the API was implicitly active whenever a token existed.
Now there's an explicit unifi.api.enabled setting that gates it:

* WifiApiController returns 503 ("API disabled") when the setting is
  off, even if a valid token is presented. Stops the endpoint from
  silently working if a token is lying around.
* Settings page exposes the toggle under the Rotate-WiFi-Passwords
  block. With it off, the token / URL / curl example are hidden.
* The form submit handles the new api_enabled boolean.

v1.6.3.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

composer.json

@@ -1,7 +1,7 @@
 {
     "name": "dashboard/unifi",
     "description": "UniFi network management, WiFi stats, and captive portal authentication for the Dashboard platform",
-    "version": "1.6.2",
+    "version": "1.6.3",
     "type": "library",
     "license": "MIT",
     "autoload": {