docs: add date input reset to React showcase

[Copilot]

Addresses feedback from #5440 to demonstrate empty string reset for date inputs across all framework showcases.

Changes

React showcase (showcases/react-showcase/src/components/form/index.tsx):

• Added value={dateinput} prop to date input (line 121) - converts from uncontrolled to controlled component
• Added setDateinput('') to reset button handler (line 258)

Vue showcase - already implements date input reset at line 60

// Before: uncontrolled date input
<DBInput
  type="date"
  onChange={(event) => setDateinput(event.target.value)}
/>

// After: controlled date input with reset capability
<DBInput
  type="date"
  value={dateinput}
  onChange={(event) => setDateinput(event.target.value)}
/>

// Reset button now clears both text and date inputs
<DBButton onClick={() => {
  setInput('reset');
  setDateinput('');  // ← added
}}>

Aligns React showcase with Angular showcase pattern where form.get('dateinput')?.setValue('') resets date fields.

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