Add proper error handling for session ID clashes

Return 409 Conflict status when user-provided connection ID already exists, instead of generic 500 error.

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

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

src/app.ts

@@ -152,6 +152,12 @@ export function createApp(storage: Storage, config: AppConfig) {
       return c.json({ code }, 200);
     } catch (err) {
       console.error('Error creating offer:', err);
+
+      // Check if it's a session code clash error
+      if (err instanceof Error && err.message.includes('already exists')) {
+        return c.json({ error: err.message }, 409);
+      }
+
       return c.json({ error: 'Internal server error' }, 500);
     }
   });