feat(wallet): cross-platform NWC connection backup via NIP-78 + parity polish

app/src/main/kotlin/com/wisp/app/nostr/Nip78.kt

@@ -128,4 +128,48 @@ object Nip78 {
     /** Extract the d-tag value from an event, or null. */
     fun extractDTag(event: NostrEvent): String? =
         event.tags.firstOrNull { it.size >= 2 && it[0] == "d" }?.get(1)
+
+    // ─── NWC connection backup ────────────────────────────────────────
+    //
+    // Cross-platform NWC URI backup per `NWC_BACKUP_PARITY.md`. iOS
+    // and Android both publish to / read from the same flat `d` tag
+    // (`nwc-wallet-backup` — no `wisp-` prefix, intentional, for
+    // cross-platform interop). Content is the raw NWC URI string,
+    // NIP-44 v2 encrypted to self.
+
+    const val NWC_BACKUP_D_TAG = "nwc-wallet-backup"
+
+    /** Build + sign a kind-30078 event carrying the encrypted NWC URI. */
+    suspend fun createNwcBackupEvent(signer: NostrSigner, uri: String): NostrEvent {
+        val encrypted = signer.nip44Encrypt(uri.trim(), signer.pubkeyHex)
+        val tags = listOf(
+            listOf("d", NWC_BACKUP_D_TAG),
+            listOf("client", "Wisp"),
+            listOf("encryption", "nip44")
+        )
+        return signer.signEvent(kind = KIND, content = encrypted, tags = tags)
+    }
+
+    /**
+     * Decrypt a kind-30078 NWC-backup event and return the raw URI.
+     * Returns null when the content is empty, decrypt fails, or the
+     * plaintext doesn't look like a `nostr+walletconnect://` URI.
+     */
+    suspend fun decryptNwcBackup(signer: NostrSigner, event: NostrEvent): String? {
+        if (event.content.isBlank()) return null
+        return try {
+            val decrypted = signer.nip44Decrypt(event.content, event.pubkey).trim()
+            if (decrypted.startsWith("nostr+walletconnect://", ignoreCase = true)) decrypted else null
+        } catch (_: Exception) {
+            null
+        }
+    }
+
+    /** Filter to fetch the user's NWC backup (single addressable event). */
+    fun nwcBackupFilter(pubkeyHex: String): Filter = Filter(
+        kinds = listOf(KIND),
+        authors = listOf(pubkeyHex),
+        dTags = listOf(NWC_BACKUP_D_TAG),
+        limit = 1
+    )
 }