fuuko/wordclock: Use 15 character long password

```cpp
    struct {
      char domain[32];
      char clientId[16];
      char user[16];
      char password[16];
    } mqtt;
```

(f637c2f39e/PersistentStorage.h)

This went unnoticed, because on NixOS, mosquitto does not validate
passwords by default.

machines/fuuko/services/wordclock-dimmer.nix

@@ -9,7 +9,7 @@
         acl = [
           "topic readwrite wordclock/color/+"
         ];
-        password = "ymfQkXcEqGuk62SH";
+        password = "ymfQkXcEqGuk62S";
       };
     };
     checkPasswords = true;