fix: improve paste support for PIN code input fields

[zhangqifan]

Description

Improves the paste functionality for PIN code input fields used in SMS and 2FA authentication flows.

Problem

When pasting a 6-digit verification code like "123456" into the PIN code input fields, only the last digit ("6") would appear in the first field. The other digits were lost.

Solution

• Add detection for multi-character paste operations
• Filter pasted content to alphanumeric characters only
• Always start filling from the first field when pasting
• Optimize performance by updating all fields at once
• Support both numeric and alphanumeric codes (e.g., 123456, A1B2C3)

Impact

This improvement benefits both:

• SignInSMSView - SMS verification code input
• SignIn2FAView - 2FA device verification code input

Both views share the same PinCodeTextField component, so the fix applies automatically to both flows.

Testing

Tested with:

• ✅ Pasting 6-digit numeric codes (123456)
• ✅ Pasting with cursor in different positions (always fills from first field)
• ✅ Pasting codes with special characters (auto-filtered to alphanumeric only)
• ✅ Manual single-character input (unchanged behavior)

Checklist

• Code follows the project's style guidelines
• Self-review of code completed
• No new warnings or errors introduced
• Changes tested with real authentication flow