fix(lite): fast retry when a server is only warming up (-28)

When the preferred lightwalletd server is reachable but warming up (JSON-RPC -28
/ "Activating best chain"), the failover treated it like a dead server and fell
through to the others, so the wallet didn't open until the next 20s retry — even
though the healthy server was ready within seconds.

Detect the warmup error during failover, flag it on the open outcome
(lastOpenWasWarmup()), and have the App retry on a short ~4s interval in that case
instead of 20s, so the wallet opens promptly once warmup clears. A unit test
covers a warming-preferred + dead-fallback open setting the flag.

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

src/app.cpp

@@ -490,11 +490,13 @@ void App::update()
         // needs no passphrase — it just loads the file and contacts the server). Running it off
         // the UI thread means an unreachable server never freezes startup, and trying the other
         // default servers means one dead server no longer strands the wallet. Retried on an
-        // interval so a transient outage self-heals once a server comes back.
+        // interval so a transient outage self-heals once a server comes back — and much sooner
+        // (a few seconds) when the failure was a server merely warming up (-28), which clears fast.
         const double nowSecs = ImGui::GetTime();
+        const double retryInterval = lite_wallet_->lastOpenWasWarmup() ? 4.0 : 20.0;
         if (!lite_wallet_->walletOpen() && !lite_wallet_->openInProgress() &&
             lite_wallet_->walletExists() &&
-            (!lite_autoopen_done_ || nowSecs - lite_open_last_attempt_ > 20.0)) {
+            (!lite_autoopen_done_ || nowSecs - lite_open_last_attempt_ > retryInterval)) {
             lite_autoopen_done_ = true;
             lite_open_last_attempt_ = nowSecs;
             lite_wallet_->beginOpenExisting();