fix(e2e): use fixed header height in viewport fixture

[nmerget]

Proposed changes

Replace dynamic header height fetching with a fixed value. scrollHeight includes hidden sub-navigation items, whose height increases with each new component added to the navigation, causing inconsistent viewport sizes across test runs.

Types of changes

• Bugfix (non-breaking change that fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improvements to existing components or architectural decisions)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Further comments

🔭🐙🐈 Test this branch here: https://design-system.deutschebahn.com/core-web/review/fix-snapshot-header-height

[changeset-bot]

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