fix: validate base username length separately from clan tag (#3098)

## Description:

fix: validate base username length separately from clan tag)
## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

w.o.n

src/client/UsernameInput.ts

@@ -147,8 +147,8 @@ export class UsernameInput extends LitElement {
   }
 
   private validateAndStore() {
-    // Prevent empty username even if clan tag is present
-    if (!this.baseUsername.trim()) {
+    // Validate base username meets minimum length (clan tag doesn't count)
+    if (this.baseUsername.trim().length < MIN_USERNAME_LENGTH) {
       this._isValid = false;
       this.validationError = translateText("username.too_short", {
         min: MIN_USERNAME_LENGTH,