fix(rescan): probe the node's block range off the UI thread

Finding the lowest block a node still has (Settings > Rescan, and the
automatic reconcile after a bootstrap) ran a binary search of about 42
synchronous getblockhash/getblock calls on the UI thread, each waiting
for the RPC connection behind any wallet scan (10-20 s on a big wallet)
and each with a 30 s timeout. The window froze for that long.

It now runs as one task on the wallet RPC worker, bounded to 90 s, and
the answer comes back on the UI thread; meanwhile the dialog shows
"Checking which blocks..." with Rescan disabled. An answer from an
earlier connection counts as "could not tell", and one for an earlier
opening of the dialog is dropped. The search probes block 1 first, so a
full-history node answers in one probe, and it answers "could not tell"
at once while a runtime rescan holds the lane.

Fixed along the way: any probe error (timeout, busy node) read as "block
missing", so a busy node could get a bogus "bootstrapped, rescan from
the tip"; and a non-string getblockhash reply never called back, leaving
the dialog on "Checking..." for good. Only the daemon's "can't read
block" error now counts as missing; anything else means "could not
tell".

User-visible:
- after a bootstrap, a probe that cannot tell is tried up to 6 times,
  30 s apart and only while the node is not busy, then a toast points
  to Settings > Rescan, instead of falling back to the -rescan
  restart, which fails on a bootstrapped node;
- the Settings Rescan dialog warns when the probe could not tell.

Two new strings, translated into all eight languages.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

res/lang/es.json

@@ -69,6 +69,7 @@
     "appx_blockchain_maintenance_in_progress": "Ya hay una operación de mantenimiento de la cadena de bloques en curso.",
     "appx_blockchain_rescan_complete": "Reescaneo de la cadena de bloques completado",
     "appx_bootstrap_complete_reconciling": "Arranque inicial completado; reconciliando tu cartera con los nuevos datos de la cadena.",
+    "appx_bootstrap_reconcile_probe_failed": "No se pudo reconciliar tu cartera con los datos de la cadena del bootstrap: el nodo estuvo demasiado ocupado para comprobar qué bloques tiene. Ejecuta Re-escanear desde Ajustes cuando se haya puesto al día.",
     "appx_cancel": "Cancelar",
     "appx_cleaning_up": "Limpiando...",
     "appx_confirm_your_backup": "Confirma tu copia de seguridad",
@@ -1597,6 +1598,7 @@
     "rescan_bootstrapped_msg": "Tu nodo se inicializó con un bootstrap, por lo que los bloques anteriores a la instantánea no están en disco y un reescaneo desde el génesis fallaría. Reescanea desde una altura que incluya tu instantánea para reconciliar el saldo gastado de tu cartera. Tu wallet.dat y los datos de la cadena no se eliminan.",
     "rescan_detecting": "Comprobando qué bloques tiene tu nodo en disco…",
     "rescan_from_height": "Reescanear desde la altura de bloque:",
+    "rescan_probe_failed": "No se pudo comprobar qué bloques tiene tu nodo en disco (está ocupado o una solicitud agotó el tiempo de espera). Si este nodo se inicializó con un bootstrap, cancela y vuelve a intentarlo en un minuto: el reinicio completo con -rescan de abajo falla en un nodo inicializado con bootstrap.",
     "reset_to_defaults": "Restablecer Valores",
     "restarting_after_encryption": "Reiniciando daemon después del cifrado...",
     "restore_address": "Restaurar dirección",