[@mantine/core] PinInput: Fix readOnly not preventing value changes on Delete, Backspace and paste

[lavinhoque33]

Bug

PinInput with readOnly can still be edited. readonly is set on each <input>, but PinInput handles Delete, Backspace and paste itself, and those handlers update the value without checking readOnly. Delete and Backspace clear the focused field, and pasting a valid code replaces the whole value.

Fix

In PinInput.tsx, when readOnly is set:

• Delete and Backspace in handleKeyDown no longer clear a field (Backspace also no longer moves focus back, since nothing is deleted).
• handlePaste returns after preventDefault() without setting values.

Arrow keys still move focus between fields. Typing was already blocked by the native readonly attribute.

Closes #9256

Testing

• PinInput.test.tsx: readOnly does not change value on Delete, which fails on master (['', '2', '3', '4']) and passes with the fix.
• PinInput.test.tsx: readOnly does not change value on Backspace, which fails on master and passes with the fix.
• PinInput.test.tsx: readOnly does not change value on paste, which fails on master and passes with the fix.
• PinInput.test.tsx: readOnly keeps arrow key focus navigation guards that navigation still works.
• PinInput suites: 98 passed. oxlint and format:write:files are clean, and root tsc --noEmit reports no @mantine/core errors.

[rtivital]

🤖 Beep boop, Claude (Fable 5.1) speaking! Pull request is perfect, no changes needed! Verified label added.

[rtivital]

Thanks!