fix(lite): surface auto-open failures instead of a silent disconnected spinner

The startup auto-open of an existing lite wallet discarded openWallet()'s result,
so when initialize_existing failed (e.g. the lightwalletd server is unreachable)
the UI just showed a "disconnected" spinner with no reason — and DEBUG_LOGF is
compiled out of release builds, so there was no way to see why. Capture the
failure: store the reason, show it in the Network tab status line (in place of
"no wallet open"), and raise a notification. Cleared once a wallet opens.

This doesn't change open behaviour — it makes a stuck open diagnosable.

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

src/app.h

@@ -157,6 +157,8 @@ public:
     config::Settings* settings() { return settings_.get(); }
     // Lite wallet controller (non-null only in lite builds with a linked backend).
     wallet::LiteWalletController* liteWallet() { return lite_wallet_.get(); }
+    // Reason the lite wallet failed to auto-open this session (empty if none / opened OK).
+    const std::string& liteOpenError() const { return lite_open_error_; }
     // Show the lite send-time unlock modal (called when a spend is attempted on a locked wallet).
     void requestLiteUnlock() { lite_unlock_prompt_ = true; }
     // (Re)build the lite controller from current settings so a changed lite-server selection
@@ -442,6 +444,9 @@ private:
     // 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;
+    // Reason an existing lite wallet failed to auto-open (e.g. server unreachable). Surfaced in
+    // the UI so a stuck "disconnected" state isn't silent; cleared once a wallet opens.
+    std::string lite_open_error_;
     // Lite first-run welcome prompt: dismissed for the session once the user picks an action.
     bool lite_firstrun_dismissed_ = false;
     // Lite send-time unlock: set to show the unlock modal when a spend is attempted while locked.