feat(wallet): add SSSS backup for wallet seed phrase

Adds ability to backup wallet seed phrase to Matrix SSSS:
- WalletBackupService interface and implementation
- New BACKUP_TO_MATRIX step in wallet setup flow
- Recovery key input UI with FLAG_SECURE
- Graceful handling of invalid keys and missing SSSS setup

Users can now:
1. Write down seed phrase manually (existing)
2. Encrypt and store in Matrix account with recovery key

The backup is encrypted with the same key used for
cross-signing and message backup (SSSS).

features/wallet/api/src/main/kotlin/io/element/android/features/wallet/api/backup/WalletBackupService.kt

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2026 Sulkta Coop.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package io.element.android.features.wallet.api.backup
+
+/**
+ * Service for backing up and restoring wallet seed phrases using Matrix SSSS.
+ *
+ * The backup is encrypted with the user's Matrix recovery key and stored
+ * in their account data, so it follows them across devices.
+ */
+interface WalletBackupService {
+    /**
+     * The secret name used to store the wallet seed in SSSS.
+     */
+    companion object {
+        const val SECRET_NAME = "com.sulkta.cardano.wallet_seed"
+    }
+
+    /**
+     * Backup the wallet seed phrase to Matrix SSSS.
+     *
+     * @param recoveryKey The Matrix recovery key (base58 encoded)
+     * @param mnemonic The wallet seed phrase to backup
+     * @return Success or error
+     */
+    suspend fun backupSeed(recoveryKey: String, mnemonic: List<String>): Result<Unit>
+
+    /**
+     * Restore a wallet seed phrase from Matrix SSSS.
+     *
+     * @param recoveryKey The Matrix recovery key
+     * @return The mnemonic words if found, null if no backup exists
+     */
+    suspend fun restoreSeed(recoveryKey: String): Result<List<String>?>
+
+    /**
+     * Check if a wallet backup exists in SSSS.
+     *
+     * This can be called with the recovery key to verify a backup is present.
+     *
+     * @param recoveryKey The Matrix recovery key
+     * @return True if a backup exists, false otherwise
+     */
+    suspend fun hasBackup(recoveryKey: String): Result<Boolean>
+}