fix(ui): suppress app-unreachable overlay while ElectrumX sync screen shows

When ElectrumX is still building its index (or waiting on the Bitcoin node),
AppSessionFrame shows a sync 'pre UI'. The iframe-blocked fallback ('App not
reachable / retrying') was not gated on electrsSync, so it painted over the
sync screen and read as a hard connection error. Gate it on !electrsSync,
mirroring the iframe's own guard.

Also harden the lifecycle health probe: container_health used jq '// "unknown"',
which only catches null/false — an empty-string health (a brief window under
load) rendered as a blank 'bad health: X is '. Map empty to 'unknown' so the
retry loop keeps waiting instead of failing on a transient.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

neode-ui/src/views/appSession/AppSessionFrame.vue

@@ -60,9 +60,12 @@
       />
     </div>
 
-    <!-- Iframe blocked fallback -->
+    <!-- Iframe blocked fallback. Suppressed while the ElectrumX sync screen
+         (the "pre UI") is showing: a still-syncing Electrum server isn't
+         reachable yet, so the "App not reachable / retry" overlay would just
+         paint over the sync progress and read as a hard error. -->
     <Transition name="content-fade">
-      <div v-if="iframeBlocked" class="absolute inset-0 z-10 flex flex-col items-center justify-center">
+      <div v-if="iframeBlocked && !electrsSync" class="absolute inset-0 z-10 flex flex-col items-center justify-center">
         <div class="text-center px-8">
           <div class="w-16 h-16 mx-auto mb-4 rounded-2xl bg-white/5 border border-white/10 flex items-center justify-center">
             <svg class="w-8 h-8 text-white/40" fill="none" stroke="currentColor" viewBox="0 0 24 24">