feat(wallet): per-wallet data isolation foundation (P1)

First phase of multi-wallet support: keep each wallet's data separate so loading
a different wallet no longer shows the previous one's data, and lay the metadata
groundwork for the wallet-files list.

- Address book scoped per wallet: AddressBookEntry gains a "scope" ("global" or a
  wallet-identity hash); the Contacts tab shows global + current-wallet contacts
  (App::activeWalletIdentityHash) with a "show in every wallet" toggle + globe
  badge. Legacy entries migrate to "global". Scope-aware de-dup allows the same
  address across different wallets.
- Wallet metadata index (data/wallet_index -> wallets.json): file-keyed cache of
  balance / address count / identity / size / last-opened / synced-here, since
  those can't be read off a wallet.dat without loading it. Populated on connect +
  address refresh (change-detecting upsert). Plus an active_wallet_file setting
  for the -wallet=<name> switch coming in P2.

Unit-tested (testAddressBookScope, testWalletIndex): migration, visibility filter,
scope-aware de-dup, upsert change-detection, save/reload round-trip.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/data/wallet_index.h

@@ -0,0 +1,68 @@
+// DragonX Wallet - ImGui Edition
+// Copyright 2024-2026 The Hush Developers
+// Released under the GPLv3
+
+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace dragonx {
+namespace data {
+
+/**
+ * @brief Cached, file-keyed metadata for one wallet file.
+ *
+ * A wallet.dat on disk only reveals its size + mtime; balance and address count require the daemon
+ * to have loaded it. So those are cached here after each load and shown in the wallet-files list
+ * (P2) even when the wallet isn't the active one. walletIdentityHash bridges file -> per-wallet data
+ * (address book, tx history) so the right caches can be selected before/after a switch.
+ */
+struct WalletIndexEntry {
+    std::string fileName;               // plain wallet filename in the datadir (e.g. "wallet.dat")
+    std::string displayName;            // user-facing name (defaults to fileName)
+    std::string walletIdentityHash;     // address-derived identity of the last load; "" = unknown
+    double      cachedBalance = -1.0;   // last-known total balance; < 0 = unknown (never opened)
+    long long   cachedAddressCount = -1;// < 0 = unknown
+    long long   lastOpenedEpoch = 0;    // unix seconds of last open; 0 = never opened
+    long long   sizeBytesAtLastOpen = 0;
+    bool        syncedHere = false;     // loaded in this datadir before -> catch-up on switch (no full rescan)
+};
+
+/**
+ * @brief Persistent wallet-files metadata index (wallets.json, in the config dir).
+ *
+ * Keyed by wallet file name. Populated after each load (P1b) and read by the wallet-files list (P2).
+ * Also remembers extra folders the user added to scan for wallet files. Not encrypted — it holds
+ * only file names + coarse metadata (no keys/addresses).
+ */
+class WalletIndex {
+public:
+    bool load();
+    bool save();
+    static std::string getDefaultPath();
+
+    const std::vector<WalletIndexEntry>& entries() const { return entries_; }
+    const std::vector<std::string>& extraFolders() const { return extra_folders_; }
+
+    /**
+     * @brief Insert or update the entry for e.fileName.
+     * @return true if a new entry was added or any field changed (so the caller can skip save()).
+     */
+    bool upsert(const WalletIndexEntry& e);
+
+    /** @brief Find the entry for a wallet file, or nullptr. */
+    const WalletIndexEntry* find(const std::string& fileName) const;
+
+    /** @brief Add/remove an extra scan folder. Returns true if the set changed. */
+    bool addExtraFolder(const std::string& dir);
+    bool removeExtraFolder(const std::string& dir);
+
+private:
+    std::vector<WalletIndexEntry> entries_;
+    std::vector<std::string> extra_folders_;
+    std::string file_path_;
+};
+
+} // namespace data
+} // namespace dragonx