Allow to customize initial Popover placement

[acelaya]

Add a new placement?: 'above' | 'below' property to the Popover, so that it is possible to customize the default placement of the popover relative to the anchor.

Up until now, popovers would always render below the anchor when there's enough space.

The component can still decide to render in the opposite placement if selected one does not allow for it to render inside the viewport.

popover-placement-2025-06-17_10.01.03.mp4

Test steps

1. Go to http://localhost:4001/feedback-popover
2. Scroll down to the placement property doc, which has an example of above and below popovers.
3. While the buttons are more or less in the middle, popovers should show in the placement indicated by every button.
4. If the buttons are close to the top or bottom, their placements may change.
5. Resize the window so that the popovers do not fit neither above or below.
6. They should now show in the direction where there's more available space, even if they don't fit.

Warning

Ideally, when the popover does not fit neither above or below, it should re-position and overlap the anchor element, to make sure it is kept in the viewport.
That is not part of this PR, and will be addressed as part of #1921

TODO

• Ensure placement falls back only if the popover does not fit in available space, not if there's more space in the opposite direction.
• Add tests.
• Document new prop.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7109fdc) to head (e1c48ce).
Report is 2 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           70        70           
  Lines         1314      1319    +5     
  Branches       484       489    +5     
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+ Hits          1314      1319    +5     

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[acelaya]

Tests were a bit flaky without this, due to the default vitest resolution.

This is the viewport we have set in the rest of the projects, but it was overlooked here because it was the first project migrated to vitest.

[acelaya]

Adjustment needed for the new viewport size used in tests. The viewport is now wider so the text needs to be longer.

[robertknight]

LGTM