feat(mobile): crypto + secret-store + session-storage adapters

apps/mobile/lib/cryptoBackend.ts

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+import * as s from 'react-native-libsodium';
+import type { CryptoBackend } from '@chat-app/shared/crypto';
+
+// react-native-libsodium re-exports libsodium-wrappers' API shape, so
+// this adapter is the synchronous twin of the desktop one
+// (`apps/desktop/src/lib/cryptoBackend.ts`). No WASM warm-up gate is
+// needed — the native module is ready as soon as the module loads.
+export function createLibsodiumBackend(): CryptoBackend {
+  return {
+    name: 'react-native-libsodium',
+    nonceLength: s.crypto_box_NONCEBYTES,
+    publicKeyLength: s.crypto_box_PUBLICKEYBYTES,
+    privateKeyLength: s.crypto_box_SECRETKEYBYTES,
+    secretboxKeyLength: s.crypto_secretbox_KEYBYTES,
+    secretboxNonceLength: s.crypto_secretbox_NONCEBYTES,
+    randomBytes: (n) => s.randombytes_buf(n),
+    generateKeyPair: () => {
+      const kp = s.crypto_box_keypair();
+      return { publicKey: kp.publicKey, privateKey: kp.privateKey };
+    },
+    box: (plaintext, nonce, recipientPublicKey, senderPrivateKey) =>
+      s.crypto_box_easy(plaintext, nonce, recipientPublicKey, senderPrivateKey),
+    boxOpen: (ciphertext, nonce, senderPublicKey, recipientPrivateKey) =>
+      s.crypto_box_open_easy(ciphertext, nonce, senderPublicKey, recipientPrivateKey),
+    secretbox: (plaintext, nonce, key) => s.crypto_secretbox_easy(plaintext, nonce, key),
+    secretboxOpen: (ciphertext, nonce, key) => s.crypto_secretbox_open_easy(ciphertext, nonce, key),
+  };
+}

apps/mobile/lib/secretStore.ts

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+import { Buffer } from 'buffer';
+import * as SecureStore from 'expo-secure-store';
+import type { SecretStore } from '@chat-app/shared/auth';
+
+// SecretStore contract uses Uint8Array values; SecureStore only takes
+// strings, so we base64 at the boundary. iOS Keychain max value size
+// is generous (a few MB); private keys are 32 bytes so we are well
+// within limits.
+export const secretStore: SecretStore = {
+  async getSecret(key) {
+    const v = await SecureStore.getItemAsync(key);
+    return v ? new Uint8Array(Buffer.from(v, 'base64')) : null;
+  },
+  async setSecret(key, value) {
+    await SecureStore.setItemAsync(key, Buffer.from(value).toString('base64'));
+  },
+  async removeSecret(key) {
+    await SecureStore.deleteItemAsync(key);
+  },
+};

apps/mobile/lib/sessionStorage.ts

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+import AsyncStorage from '@react-native-async-storage/async-storage';
+
+// supabase-js v2 accepts any object with async getItem / setItem /
+// removeItem returning Promise<string | null> / Promise<void>. RN's
+// AsyncStorage matches that shape for shape; we just re-export it
+// under a name that signals intent at the call site.
+export const sessionStorage = AsyncStorage;