Fix positioning of vac-room-header

[thierbig]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

N/A

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g., fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing
  • Note: You should verify this by running the project's tests. If any tests fail because of this change, you may need to adjust them.

If adding a new feature, the PR's description includes:

• N/A

Other information:

This PR addresses an issue where the .vac-room-header CSS property position: absolute; was causing layout problems by overlapping or obstructing other page elements, particularly static headers in user applications. By changing this to position: fixed;, the chat component's header will be more contained within the chat module itself, reducing the risk of it interfering with other UI components.