feat: add keepMounted prop

[pradel]

Closes #233. Supersedes #516.

Problem

When the modal is closed it is unmounted from the DOM, so any state held by its content (e.g. an uncontrolled input value) is lost.

Solution

New optional keepMounted?: boolean prop (default false). When enabled, the portal stays mounted while closed:

• The modal is still animated out, then hidden with a visibility: hidden transition delayed by animationDuration, so it is removed from the a11y tree/tab order and cannot be clicked while hidden. Reopening reveals it instantly.
• onAnimationEnd still fires on close.
• The focus trap is unmounted while hidden, so focus is restored on close and set again on open.
• Scroll lock, esc handling and overlay clicks are unchanged and stay disabled while the modal is not open.

No new state or effects are needed: mounting is driven by open || keepMounted, and hiding is CSS-driven.

Tests

• modal stays mounted and hidden when closed
• shown again on reopen
• DOM state (input value) preserved across close/reopen
• onAnimationEnd still fires without unmounting
• esc does not close the hidden modal

Also adds a "Keeping the modal mounted" docs section with a live example.

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�[2m Test Files �[22m �[1m�[31m1 failed�[39m�[22m�[2m | �[22m�[1m�[32m1 passed�[39m�[22m�[90m (2)�[39m
�[2m Tests �[22m �[1m�[32m3 passed�[39m�[22m�[90m (3)�[39m
�[2m Start at �[22m 22:23:54
�[2m Duration �[22m 228ms�[2m (transform 98ms, setup 0ms, import 88ms, tests 5ms, environment 0ms)�[22m 63 passed, pass: All 57 files are correctly formatted (244ms, 10 threads)
pass: Found no warnings, lint errors, or type errors in 36 files (331ms, 10 threads) clean, website build passes, size 2.8/2.9 kB ESM and 3.51/3.6 kB CJS.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.06%. Comparing base (b69f310) to head (8ec1fb7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #577      +/-   ##
==========================================
+ Coverage   80.69%   81.06%   +0.37%     
==========================================
  Files           7        7              
  Lines         202      206       +4     
  Branches       74       78       +4     
==========================================
+ Hits          163      167       +4     
  Misses         29       29              
  Partials       10       10              

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