feat(wallet): store Cardano address in Matrix account data for discovery

Implements public Cardano address directory using Matrix account data:

Publishing (write side):
- After wallet creation, import, or SSSS restore, the Cardano address
  is written to the user Matrix account data
- Key: com.sulkta.cardano.address
- Content: { "address": "addr1..." }
- This is public/unencrypted for discovery by other users

Lookup (read side):
- When entering a Matrix user in /pay, their account data is checked
- If they have a linked Cardano address, it auto-fills the recipient
- UI shows "Address loaded from @username profile ✓" when found
- Shows "@username has not linked a wallet" if not found
- Graceful fallback to manual address entry

New files:
- CardanoAddressService interface (wallet:api)
- DefaultCardanoAddressService implementation (wallet:impl)

Updated:
- WalletSetupPresenter: calls publishAddress after all wallet setup paths
- PaymentEntryPresenter: looks up recipient address from Matrix
- PaymentEntryState: added Resolving and Found states
- PaymentEntryView: shows lookup progress and result cards

features/wallet/api/src/main/kotlin/io/element/android/features/wallet/api/address/CardanoAddressService.kt

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2026 Sulkta Coop.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package io.element.android.features.wallet.api.address
+
+import io.element.android.libraries.matrix.api.core.UserId
+
+/**
+ * Service for managing Cardano addresses in Matrix account data.
+ *
+ * This allows users to publish their Cardano address so other users can
+ * look it up for payments - like a public address directory baked into Matrix.
+ *
+ * Account data key: `com.sulkta.cardano.address`
+ * Content format: `{ "address": "addr1..." }`
+ */
+interface CardanoAddressService {
+    /**
+     * Publish the user's Cardano address to their Matrix account data.
+     * This is public data, not encrypted.
+     *
+     * @param address The Cardano address to publish
+     * @return Result indicating success or failure
+     */
+    suspend fun publishAddress(address: String): Result<Unit>
+
+    /**
+     * Look up another user's Cardano address from their Matrix account data.
+     *
+     * @param userId The Matrix user ID to look up
+     * @return The user's Cardano address if published, null if not found
+     */
+    suspend fun lookupAddress(userId: UserId): Result<String?>
+
+    companion object {
+        const val ACCOUNT_DATA_TYPE = "com.sulkta.cardano.address"
+    }
+}
+
+/**
+ * Result of a Cardano address lookup.
+ */
+sealed interface AddressLookupResult {
+    /** Address was found and retrieved successfully */
+    data class Found(val address: String, val userId: UserId) : AddressLookupResult
+
+    /** User has no Cardano address linked */
+    data class NotLinked(val userId: UserId) : AddressLookupResult
+
+    /** Lookup failed due to an error */
+    data class Error(val userId: UserId, val message: String) : AddressLookupResult
+}