Kiosk: schedule client (conditional GET + atomic cache) and engine

Adds api_client support for GET /licenses/{key}/schedule with If-None-Match
and target-payload parsing (version/type/daily/weekly + legacy blocks), an
atomic on-disk schedule cache, a pure schedule engine (half-open blocks,
midnight self-wrap for daily, Sunday->Monday spillover for weekly, invalid
blocks skipped with a warning), and a background ScheduleManager that loads
the cache offline and pulls on boot and every 5 minutes. Adds engine/cache
tests against the shared docs/fixtures, including 304, corrupt cache, and
offline cases.

src/timezone_helper.h

@@ -27,6 +27,10 @@ std::string formatUtcToTimezone(time_t utc_time, const std::string& iana_code, c
 // timezone is empty.
 int currentLocalMinutes(const std::string& iana_code);
 
+// Weekday right now in `iana_code`, 0=Monday .. 6=Sunday. Falls back to UTC when
+// the timezone is empty.
+int currentLocalWeekday(const std::string& iana_code);
+
 // Gets current UTC timestamp as ISO string
 std::string getUtcTimestampIso();