Fix VssStoreIdProvider cache to be per walletIndex

This change ensures that VSS store ID derivation maintains separate
caches for each wallet index, preventing collisions when managing
multiple wallets within the same application instance.

Changes:
- Replace single cachedStoreId with cachedStoreIds map keyed by walletIndex
- Update getVssStoreId() to accept walletIndex parameter (defaults to 0)
- Add clearCache() method to clear all cached store IDs
- Add clearCache(walletIndex) method to clear cache for specific wallet
- Update LightningService to pass walletIndex when getting store ID
- Enhance logging to include walletIndex

This matches the improvements from bitkit-ios PR #202, commit 91b47ba.

Author: claude

app/src/main/java/to/bitkit/data/backup/VssStoreIdProvider.kt

@@ -13,13 +13,13 @@ class VssStoreIdProvider @Inject constructor(
     private val keychain: Keychain,
 ) {
     @Volatile
-    private var cachedStoreId: String? = null
+    private var cachedStoreIds: MutableMap<Int, String> = mutableMapOf()
 
-    fun getVssStoreId(): String {
-        cachedStoreId?.let { return it }
+    fun getVssStoreId(walletIndex: Int = 0): String {
+        cachedStoreIds[walletIndex]?.let { return it }
 
         return synchronized(this) {
-            cachedStoreId?.let { return it }
+            cachedStoreIds[walletIndex]?.let { return it }
 
             val mnemonic = keychain.loadString(Keychain.Key.BIP39_MNEMONIC.name) ?: throw ServiceError.MnemonicNotFound
             val passphrase = keychain.loadString(Keychain.Key.BIP39_PASSPHRASE.name)
@@ -30,12 +30,24 @@ class VssStoreIdProvider @Inject constructor(
                 passphrase = passphrase,
             )
 
-            Logger.info("VSS store id: '$storeId'", context = TAG)
-            cachedStoreId = storeId
+            Logger.info("VSS store id: '$storeId' for walletIndex: $walletIndex", context = TAG)
+            cachedStoreIds[walletIndex] = storeId
             storeId
         }
     }
 
+    fun clearCache() {
+        synchronized(this) {
+            cachedStoreIds.clear()
+        }
+    }
+
+    fun clearCache(walletIndex: Int) {
+        synchronized(this) {
+            cachedStoreIds.remove(walletIndex)
+        }
+    }
+
     companion object {
         private const val TAG = "VssStoreIdProvider"
     }

app/src/main/java/to/bitkit/services/LightningService.kt

@@ -104,7 +104,7 @@ class LightningService @Inject constructor(
 
         Logger.debug("Building node…")
 
-        val vssStoreId = vssStoreIdProvider.getVssStoreId()
+        val vssStoreId = vssStoreIdProvider.getVssStoreId(walletIndex)
 
         ServiceQueue.LDK.background {
             node = try {