feat(printer): add smart-plug power switch button (Issue #103); fix(update): stable update check missing behind prereleases (Issue #104)

Issue #103: the printer has no MQTT-level command to power off or enter
standby, so users on a separate-room setup have to physically walk over
or use a smart plug (e.g. Tasmota) manually. Added per-printer
power_on_url/power_off_url/power_status_url config (Settings > Power
Switch) and a power button with a live on/off indicator on each printer's
card in the Printers grid - plain HTTP GET calls, no Moonraker
device_power dependency.

Issue #104: the stable-release update check only ever requested the
single newest Gitea release (limit=1) regardless of type. Since
nightly/dev prereleases publish far more often than stable ones, that
newest release is almost always a prerelease, so the "not a prerelease"
filter found nothing and reported "no stable releases found" even
though a newer stable release existed further back in the list.
Fixed by requesting enough releases (limit=20) to look past a run of
prereleases.

Also fixes a related pre-existing bug surfaced while testing #103's
config: configparser's default string interpolation rejected any
config.ini value containing a literal '%' (ValueError: invalid
interpolation syntax) - this broke saving Tasmota-style power URLs
(cmnd=Power%20on) and would have broken any other value with a '%'
character. Fixed globally with interpolation=None on every
ConfigParser() instantiation in config_loader.py and
kobrax_moonraker_bridge.py.

env_loader.py

@@ -46,6 +46,9 @@ USERNAME         = get("MQTT_USERNAME",    "")
 PASSWORD         = get("MQTT_PASSWORD",    "")
 MODE_ID          = get("MODE_ID",          "")
 DEVICE_ID        = get("DEVICE_ID",        "")
+POWER_ON_URL     = get("POWER_ON_URL",     "")
+POWER_OFF_URL    = get("POWER_OFF_URL",    "")
+POWER_STATUS_URL = get("POWER_STATUS_URL", "")
 DEFAULT_AMS_SLOT = get("DEFAULT_AMS_SLOT", "auto")
 AUTO_LEVELING            = int(get("AUTO_LEVELING", "1"))
 VIBRATION_COMPENSATION   = int(get("VIBRATION_COMPENSATION", "0"))