frontend: Fix Streamlabs sync offset overflow

[cuishuang]

Description

The Streamlabs importer stores the calculated audio sync offset in an int:

int sync = (int)(in_sync["sec"].number_value() * 1000000000 +
                 in_sync["nsec"].number_value());

This truncates the nanosecond offset to 32 bits. Sync offsets above approximately 2.147 seconds, or below approximately -2.147 seconds, can therefore be converted incorrectly before being written to the imported scene collection.

Calculate the offset using int64_t instead.

Motivation and Context

OBS stores source audio sync offsets as 64-bit nanosecond values through obs_source_set_sync_offset() and obs_data_get_int().

The OBS audio settings UI permits sync offsets up to 20,000 ms, so valid Streamlabs offsets such as 3, 5, 10, or 20 seconds must be preserved during import.

This change keeps the calculation in 64-bit arithmetic. json11 does not provide an int64_t constructor, so the final intermediate JSON value is explicitly represented as double; the supported range is well within the exact integer range of an IEEE-754 double.

How Has This Been Tested?

• Verified the old and new calculations with Apple Clang for 3, 5, 10, and 20 second offsets.
• Verified negative offsets, including -3 seconds.
• Verified that a 3 second offset serializes as 3000000000.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[PatTheMav]

Please complete the checklist. Per our contribution guidelines (that you checked to have read) you can check items that do not apply.

[cuishuang]

Please complete the checklist. Per our contribution guidelines (that you checked to have read) you can check items that do not apply.

@PatTheMav Done, thank you for the clarification.