feat: disable START button until route is connected

[nitininhouse]

Summary

Disables the START button until the player connects a box to a sink.

Changes

• event_button.gd: Button starts disabled and grayed out, added enable_start() function
• SinkClick.gd: Calls enable_start() when a route is connected

Behavior

1. Game starts → START button is disabled (grayed out)
2. Player clicks box → clicks sink → route connected
3. START button becomes enabled (normal color)
4. Player can now click START

Closes #97

[knative-prow]

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This pull-request has been approved by: nitininhouse
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