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>

tests/lifecycle/remote-lifecycle.sh

@@ -250,7 +250,7 @@ container_health() {
   health=$(
     ARCHY_RPC_TIMEOUT="${ARCHY_HEALTH_RPC_TIMEOUT:-20}" \
       rpc_result container-health "$(jq -nc --arg app "$app" '{app_id:$app}')" \
-      | jq -r --arg app "$app" '.[$app] // "unknown" | ascii_downcase'
+      | jq -r --arg app "$app" '(.[$app] // "") | if . == "" then "unknown" else ascii_downcase end'
   ) || health=unknown
   if [[ "$app" == "indeedhub" && "$health" != "healthy" ]] && probe_launch "$app" >/dev/null 2>&1; then
     health=healthy