feat(lite): auto-open existing wallet on startup + gate full-node RPC refreshes

Auto-open: on the first update() tick (kept off init() so a slow
initialize_existing network call can't freeze startup before the window), if a
wallet file exists, open it. initialize_existing needs no passphrase — it loads
the file; a previously-synced + saved wallet resumes from its height (fast)
instead of rescanning from the checkpoint. Adds LiteWalletController::walletExists()
(bridge.walletExists on the connection's chain) + a chainName_ member.

RPC-refresh gating: the earlier connected=walletOpen() fix (so the wallet UI is
enabled in lite) had a side effect — the full-node periodic + per-page RPC
refreshes (mining/balance/peers/txs, and setCurrentPage's immediate refresh)
gate on state_.connected, so they began firing in lite and failing
("X error: Not connected"). Re-gate those on ACTUAL RPC connectivity
(rpc_ && rpc_->isConnected()) instead of the lite proxy. Full-node is unchanged
(state_.connected ⟺ rpc connected there); lite no longer issues any RPC.

Runtime-verified in WSLg with a pre-seeded wallet: app auto-opens (Starting
Mempool + sync begins), and "Not connected" / getMiningInfo / RPC-connect noise
all drop to 0 — a fully clean lite run. tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/wallet/lite_wallet_controller.h

@@ -127,6 +127,9 @@ public:
     void setPersistCallback(std::function<void()> callback) { persist_ = std::move(callback); }
 
     bool walletOpen() const { return walletOpen_; }
+    // True if a wallet file already exists on disk for this chain. Use to auto-open on startup
+    // (open via openWallet({}) needs no passphrase — initialize_existing just loads the file).
+    bool walletExists() const;
     const WalletBackendStatus& status() const { return status_; }
     LiteWalletLifecycleAvailability availability() const { return lifecycle_.availability(); }
 
@@ -214,6 +217,7 @@ private:
     // sync_status separately), so concurrent execute() calls cannot corrupt state. A coarse
     // mutex would instead serialize sync against syncstatus polling and defeat the design.
     std::shared_ptr<LiteClientBridge> bridge_;
+    std::string chainName_;  // backend chain id (for walletExists); from connection settings
     LiteWalletLifecycleService lifecycle_;
     LiteWalletGateway gateway_;
     LiteSyncService sync_;