fix(contacts): stable per-wallet scope so contacts don't vanish (address-hash drift)

Contacts were scoped by activeWalletIdentityHash() — a hash of the wallet's
ENTIRE address set. Creating a new receive address grows the set, changing the
hash, so every contact stamped with the old hash falls out of the scope filter
(contacts_tab.cpp:915) while still being counted — the "3 saved, 1 showing"
symptom, where only the one set to global (which bypasses the scope) survives.
It also hid scoped contacts on every startup before the daemon connected (hash
empty until addresses load).

Introduce a stable per-wallet scope id: WalletIndexEntry.scopeId ("w:"+random
hex), generated once and persisted in the wallet index (keyed by wallet file),
never recomputed from the mutable address set — so creating addresses, locking,
or disconnecting never changes it. App::activeWalletScopeId() establishes it on
first use. Contacts now scope + filter on this instead of the drifting hash. The
tx-history-cache identity (the hash's real purpose) is untouched.

Recovery for already-orphaned contacts:
- AddressBook::reattachLegacyScopes() re-attaches non-global, non-"w:" contacts
  to the active wallet's stable id; run once when there's a single known wallet
  (unambiguous attribution). Idempotent.
- The scope filter fails OPEN for legacy scopes (multi-wallet case where recovery
  can't attribute them) so no contact is ever hidden; stable "w:" scopes still
  match strictly.

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

src/data/address_book.cpp

@@ -144,6 +144,20 @@ bool AddressBook::removeEntry(size_t index)
     return save();
 }
 
+int AddressBook::reattachLegacyScopes(const std::string& scopeId)
+{
+    if (scopeId.empty()) return 0;
+    int rescoped = 0;
+    for (auto& e : entries_) {
+        if (e.isGlobal()) continue;                       // global stays global
+        if (e.scope.rfind("w:", 0) == 0) continue;        // already a stable scope
+        e.scope = scopeId;
+        ++rescoped;
+    }
+    if (rescoped > 0) save();
+    return rescoped;
+}
+
 int AddressBook::findByAddress(const std::string& address) const
 {
     for (size_t i = 0; i < entries_.size(); i++) {