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/wallet_index.h

@@ -22,6 +22,8 @@ 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
+    std::string scopeId;                // stable per-wallet id ("w:"+hex) for scoping contacts etc.;
+                                        // generated once, never recomputed from the (mutable) address set
     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