River WebSocket hardcoded to port 50509 instead of using serving port

[sanity]

Summary

The River app WebSocket connection is hardcoded to connect to port 50509, but it should dynamically connect to whatever port the HTTP page was served from.

Current Behavior

• River app served on any port (e.g., 55509)
• WebSocket tries to connect to hardcoded port 50509
• Connection fails if Freenet is not running on port 50509

Expected Behavior

• WebSocket should connect to the same port that served the HTTP page
• Use window.location.port or similar to dynamically determine the correct port

Test Case

1. Start Freenet on non-default port: freenet network --port 55509
2. Navigate to River app: http://127.0.0.1:55509/v1/contract/web/BcfxyjCH4snaknrBoCiqhYc9UFvmiJvhsp5d4L5DuvRa/
3. Observe WebSocket connection error trying to connect to port 50509

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