fix: Add gas budget tracker with configurable limits

[ledgerpilot]

Closes #5

What changed

This fix introduces a GasTracker utility class to manage cumulative gas spending with hourly and daily limits, pausing operations when budgets are exhausted. The GasTracker ensures thread-safe operations and uses mockable time for robust testing. It provides methods to record gas usage, check if a transaction can be sent, and determine if the tracker is currently paused. New pytest tests are i

Files modified

• switchboard/gas_tracker.py
• tests/test_gas_tracker.py

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Draft PR — please review before merging.