fix: reject start=0 on 1-based badge endpoints, fix demo goal calls

[cyberjunky]

Follow-up to #441.

• get_badge_challenges, get_available_badge_challenges and get_non_completed_badge_challenges now reject start=0 with a ValueError, like get_inprogress_virtual_challenges already did. Garmin rejects 0 on these endpoints anyway (400 - start should > 0, checked against the live API).
• demo.py goal options passed start=0, which feat: update get_goals method to use 1-based indexing #441 now rejects; they use start_badge (1) now.
• get_adhoc_challenges is left as is: the endpoint accepts start=0.
• Unit tests for the zero-start rejection.

Summary by CodeRabbit

• Bug Fixes
  • Badge, challenge, and goal menus now use consistent one-based starting positions, improving which items are displayed.
  • Badge-challenge requests now require a positive starting position. Requests using zero return a validation error.

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• demo.py
• garminconnect/__init__.py
• tests/test_garmin_unit.py

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