🎨 Palette: Enhance Modal accessibility and keyboard support

[google-labs-jules]

This PR addresses critical accessibility gaps in the Modal component. It adds standard ARIA attributes (role="dialog", aria-modal, aria-labelledby) and keyboard support (closing on Escape key), improving the experience for screen reader users and keyboard navigators. It also locks body scroll when the modal is open.

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PR created automatically by Jules for task 7117747166083028368 started by @Devasy23

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.42%. Comparing base (22f39f1) to head (d5f35f2).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #216   +/-   ##
=======================================
  Coverage   74.42%   74.42%           
=======================================
  Files          17       17           
  Lines        1693     1693           
  Branches      158      158           
=======================================
  Hits         1260     1260           
  Misses        381      381           
  Partials       52       52           

Components	Coverage Δ
Authentication System	75.45% <ø> (ø)
Expense Management	70.64% <ø> (ø)
Group Management	76.20% <ø> (ø)
User Management	97.16% <ø> (ø)
Backend Core	69.41% <ø> (ø)

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