[EuiFilePicker] Migrate from class to function component

[weronikaolejniczak]

EuiFilePicker provides a file upload input with drag-and-drop support and visual feedback.

• File: components/form/file_picker/file_picker.tsx
• Class size: 262 lines
• State: promptText, isHoveringDrop
• Lifecycle: None

Migration notes:

State-only, no lifecycle. Drag-and-drop handlers and file input refs need conversion to useRef.

Acceptance criteria:

• Convert to function component with useState and useRef
• Maintain drag-and-drop behavior
• All existing tests pass