Add panel orientation setting for upside-down mounting (#455)

Adds a display.hardware.orientation config field ("normal" / "180")
so panels mounted upside down (e.g. to put the Pi/wiring on a more
convenient side) render correctly without custom pixel_mapper_config
edits. Composes onto the existing pixel_mapper_config as a trailing
"Rotate:180" mapper, so it stays independent of any custom mapper
string (e.g. U-mapper chain layouts) already in use.

Exposed as a "Panel Orientation" dropdown in the web UI's Display
settings, validated server-side, and documented in README and
CONFIG_REFERENCE.


Claude-Session: https://claude.ai/code/session_01FakipqMDHQLpsFjTuBdSFQ

Co-authored-by: Claude <noreply@anthropic.com>

web_interface/templates/v3/partials/display.html

@@ -117,6 +117,14 @@
                     </select>
                 </div>
 
+                <div class="form-group" id="setting-display-orientation" data-setting-key="display.hardware.orientation">
+                    <label for="orientation" class="block text-sm font-medium text-gray-700">Panel Orientation{{ ui.help_tip('Rotates the rendered image to match how the panel is physically mounted.\nUse "Upside Down" if you flipped the panel 180° to move the Raspberry Pi / wiring to a more convenient side.', 'Panel Orientation') }}</label>
+                    <select id="orientation" name="orientation" class="form-control">
+                        <option value="normal" {% if main_config.display.hardware.get('orientation', 'normal') == "normal" %}selected{% endif %}>Normal</option>
+                        <option value="180" {% if main_config.display.hardware.get('orientation', 'normal') == "180" %}selected{% endif %}>Upside Down (180°)</option>
+                    </select>
+                </div>
+
                 <div class="form-group" id="setting-display-led_rgb_sequence" data-setting-key="display.hardware.led_rgb_sequence">
                     <label for="led_rgb_sequence" class="block text-sm font-medium text-gray-700">LED RGB Sequence{{ ui.help_tip('Order the panel expects color channels in.\nChange this only if reds/greens/blues look swapped. Default: RGB.', 'LED RGB Sequence') }}</label>
                     <select id="led_rgb_sequence" name="led_rgb_sequence" class="form-control">