feat(NcPopover): expose distance and skidding props

[pringelmann]

floating-vue's <Dropdown> accepts distance and skidding for nudging the popper relative to the trigger. NcPopover hardcodes distance to 10 and never passes skidding through, so consumers can't shift a popover without CSS hacks that confuse the positioning math. Defaults are preserved, so this is non-breaking.

Resolves

Needed for the redesigned app launcher in nextcloud/server#59888.

Screenshots

No visual change with defaults.

Tasks

• Add distance and skidding props
• Forward both to <Dropdown>

Checklist

• Tests included or not applicable: not applicable. Defaults match the previous hardcoded values, so existing tests still cover the default path.
• Component documentation extended/updated or not applicable: prop JSDoc added, styleguidist picks it up automatically.
• Backport to stable8 for Vue 2 or not applicable: additive and non-breaking. Happy to backport if needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.55%. Comparing base (034beb5) to head (1f07966).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8484   +/-   ##
=======================================
  Coverage   54.55%   54.55%           
=======================================
  Files         106      106           
  Lines        3439     3439           
  Branches     1002     1003    +1     
=======================================
  Hits         1876     1876           
  Misses       1322     1322           
  Partials      241      241           

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