fix: delete existing user sessions before creating new ones

Fixes issue where games failed to start on iPhone with "No active session found" error.

The problem was a UNIQUE constraint on arcade_sessions.user_id - when users
tried to start a new game, the session creation would fail silently because
they already had an existing session from a previous game.

Solution: Delete any existing sessions for the user before creating a new one.
This ensures users can start new games without being blocked by stale sessions.

Error logs showed:
- "UNIQUE constraint failed: arcade_sessions.user_id"
- "Failed to fetch session"
- "Move rejected: No active session found"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/web/src/lib/arcade/session-manager.ts

@@ -98,6 +98,10 @@ export async function createArcadeSession(
     user = newUser
   }
 
+  // Delete any existing sessions for this user (to handle UNIQUE constraint on userId)
+  // This ensures the user can start a new game session
+  await db.delete(schema.arcadeSessions).where(eq(schema.arcadeSessions.userId, user.id))
+
   const newSession: schema.NewArcadeSession = {
     roomId: options.roomId, // PRIMARY KEY - one session per room
     userId: user.id, // Use the actual database ID, not the guestId