fix: respect custom time range setting in Canvas dashboards

[djbarnwal]

• Applies allow_custom_time_range from the resolved Canvas spec to the Canvas time manager
• Passes the setting through to the Canvas comparison pill so custom comparison ranges are also hidden when disabled

Canvas dashboards parsed allow_custom_time_range: false correctly, but the frontend time manager never loaded that value from the Canvas spec. As a result, Canvas time controls kept using the default true behavior and still showed custom range options.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!