feat: unique display names + inline recover on join (v0.13.1)

Backend: migration 007 adds a case-insensitive unique index on user names
per event. join endpoint returns 409 conflict when the name is taken.
find_by_event_and_name uses LOWER() for case-insensitive recovery.

Frontend: join page handles 409 with a name-taken view — amber warning,
name-choice tips, inline PIN recovery form, and "Anderen Namen wählen"
button. Test guide updated with Steps 8 and 9.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/auth/handlers.rs

@@ -57,6 +57,14 @@ pub async fn join(
     )
     .await?;
 
+    // Reject if a user with this name (case-insensitive) already exists
+    if User::name_taken(&state.pool, event.id, display_name).await? {
+        return Err(AppError::Conflict(format!(
+            "Der Name \"{}\" ist bereits vergeben.",
+            display_name
+        )));
+    }
+
     // Generate a 4-digit PIN
     let pin: String = format!("{:04}", rand::rng().random_range(0..10000u32));
     let pin_hash =