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/app.h

@@ -201,10 +201,16 @@ public:
     data::AddressBook& addressBook() { return address_book_; }
     const data::AddressBook& addressBook() const { return address_book_; }
 
-    // Hash of the active wallet's identity (derived from its address list), used to scope
-    // per-wallet data (e.g. address-book contacts). Empty until addresses are known (pre-connect).
+    // Hash of the active wallet's identity (derived from its address list). This is the tx-history
+    // cache key — it changes when the address set changes, so DON'T scope persistent user data on it.
     std::string activeWalletIdentityHash() const;
 
+    // Stable per-wallet id ("w:"+hex) for scoping persistent per-wallet data (address-book contacts).
+    // Generated once and persisted in the wallet index (keyed by wallet file), never recomputed from
+    // the mutable address set — so creating addresses / locking / disconnecting never changes it.
+    // Empty only when there is no active wallet file. Establishes + persists the id on first call.
+    std::string activeWalletScopeId();
+
     data::WalletIndex& walletIndex() { return wallet_index_; }
     const data::WalletIndex& walletIndex() const { return wallet_index_; }