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.

web/themes/default/index.html

@@ -548,6 +548,22 @@
                 <input type="text" id="s-mode-id" placeholder="20030">
               </div>
             </div>
+            <div class="card">
+              <div class="card-title"><span>🔌</span> <span id="modal-sec-power">Power Switch</span></div>
+              <div class="modal-field">
+                <label id="lbl-power-on-url">Power-On URL</label>
+                <input type="text" id="s-power-on-url" placeholder="http://192.168.x.x/cm?cmnd=Power%20on">
+              </div>
+              <div class="modal-field">
+                <label id="lbl-power-off-url">Power-Off URL</label>
+                <input type="text" id="s-power-off-url" placeholder="http://192.168.x.x/cm?cmnd=Power%20off">
+              </div>
+              <div class="modal-field">
+                <label id="lbl-power-status-url">Status URL</label>
+                <input type="text" id="s-power-status-url" placeholder="http://192.168.x.x/cm?cmnd=Power">
+                <small id="lbl-power-hint" style="color:var(--txt2)"></small>
+              </div>
+            </div>
           </div>
 
           <!-- Drucker -->