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/app.h

@@ -422,6 +422,9 @@ private:
     // Pending send_tab callback for an in-flight lite send (delivered in update() once the
     // controller's async broadcast result arrives). Only one lite send runs at a time.
     std::function<void(bool, const std::string&)> lite_send_callback_;
+    // One-shot guard: auto-open an existing lite wallet on the first update() tick (kept off
+    // init() so a slow initialize_existing network call doesn't freeze startup before the window).
+    bool lite_autoopen_done_ = false;
     std::unique_ptr<daemon::DaemonController> daemon_controller_;
     std::unique_ptr<daemon::XmrigManager> xmrig_manager_;
     util::AsyncTaskManager async_tasks_;