feat: add verify method with cross-platform auto mode detection

[div02-afk]

Adds a verify(password, encodedHash) function to check whether a plaintext password matches an existing Argon2 encoded hash.

Changes

• iOS — implemented via Argon2Swift.verifyHashString, which auto-detects the hash mode from the encoded string
• Android — implemented via argon2kt.verify, with mode auto-detected by parsing the encoded hash prefix ($argon2id$, $argon2i$, $argon2d$)
• JS — exported as a named export alongside the existing default argon2 export
• Types — verify(password, encodedHash): Promise<boolean> added to index.d.ts
• package.json — react-native added as a peer dependency
• README — usage examples and Verify section updated

Usage

import argon2, { verify } from 'react-native-argon2';

const { encodedHash } = await argon2(password, salt);
const isValid = await verify(password, encodedHash); // true

resolves #26

[div02-afk]

@zanechua please review