#21153 next: use type-scale weight for tab overflow hint

The cmux-next Type scale check fails on TabOverflowHintView.swift:51 because the new overflow hint button uses a raw .medium font weight. This failure also appeared on UI PRs #21069, #21051, and #21053 after they incorporated the same base change.

Use Typography.system with the existing caption size and TypeScale.emphasis for the button. This keeps the hint within the shared type scale and avoids creating a fresh NSFont in applyTokens().

Validation: python3 -I scripts/cmux-next/check-swift-type-scale.py and git diff --check pass. Native build remains with CI/fleet.

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Summary by cubic

Fixes the cmux-next type scale check failure on the tab overflow hint button by using Typography.system with TypeScale.emphasis instead of a raw NSFont weight.

• Removes the need to create a fresh NSFont in applyTokens().