fix(android): don't pre-fill localhost default in ServerUrl field

User pointed out they were being forced to clear "http://localhost:8080"
before typing their actual server URL. That default came from
AuthStore.DEFAULT_BASE_URL — an internal HTTP-client fallback for
when nothing's been configured, never something a user typed.

Now: pre-fill only when the stored URL is something the user
actually saved (anything other than the default). Otherwise leave
the field empty so the placeholder ("https://minstrel.example.com")
shows through and they can just start typing.

Edit case still works: if a user already saved e.g.
"http://192.168.1.10:8080" and re-opens ServerUrl after sign-out,
the field is pre-filled with that real value for them to edit.

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

android/app/src/main/java/com/fabledsword/minstrel/auth/ui/ServerUrlScreen.kt

@@ -51,7 +51,17 @@ class ServerUrlViewModel @Inject constructor(
 ) : ViewModel() {
 
     private val internal = MutableStateFlow(
-        ServerUrlFormState(url = authStore.baseUrl.value),
+        // Pre-fill ONLY if the user has previously saved a real URL —
+        // the DEFAULT_BASE_URL placeholder (`http://localhost:8080`) is
+        // an internal HTTP-client fallback, not something a user
+        // typed. Leaving the field empty lets the OutlinedTextField's
+        // placeholder ("https://minstrel.example.com") show through
+        // so the user can just type without first having to clear.
+        ServerUrlFormState(
+            url = authStore.baseUrl.value
+                .takeIf { it != AuthStore.DEFAULT_BASE_URL }
+                .orEmpty(),
+        ),
     )
     val state: StateFlow<ServerUrlFormState> = internal.asStateFlow()