Coordinator serves the last-good snapshot within a staleness threshold

WHY: any single typed fetch failure was a failed pass, so the whole
board flipped unavailable on one dropped poll and stayed dark until the
API answered again — even though HA's coordinator machinery keeps the
last good data on a failed pass and the panel could safely keep
rendering it for a while.

The coordinator now caches every successful snapshot with its
time.monotonic stamp; while the elapsed time since the last success is
within the configured staleness threshold (CONF_SNAPSHOT_STALENESS,
seconds, default 3x the update interval = 900 s) a typed failure serves
that cache instead: the panel keeps rendering last-good data, entities
stay available, and the outage is logged as a clear warning.  Past the
threshold — or before the first success — the pass fails as before
(typed error logged and chained into UpdateFailed); a later successful
fetch refreshes the cache and the entities recover with no config-entry
reload.  The threshold rides the options flow (validated as a positive
int at least the update interval, with a dedicated error string) and
the strings files; the clock is read through a _monotonic seam so the
staleness tests drive it deterministically.

custom_components/nestquest/strings.json

@@ -23,6 +23,7 @@
           "horizon_days": "Generation horizon (days)",
           "day_rollover_time": "Day rollover time (HH:MM)",
           "panel_idle_timeout": "Panel idle timeout (seconds)",
+          "snapshot_staleness": "Last-good snapshot staleness (seconds)",
           "api_base_url": "NestQuest API base URL (http://host:port)",
           "panel_token": "Panel service token",
           "admin_user_ids": "Admin users (parents)"
@@ -33,6 +34,7 @@
       "invalid": "Invalid value.",
       "invalid_time": "Time must use 24-hour HH:MM format.",
       "invalid_url": "Enter the API base URL as a full http:// or https:// URL with a host.",
+      "invalid_staleness": "Last-good snapshot staleness must be at least the update interval.",
       "no_admins": "Select at least one admin user; an empty allowlist would lock everyone out.",
       "invalid_admin": "Invalid admin selection."
     }