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/blueprints/api_v3.py

@@ -796,7 +796,7 @@ def save_main_config():
                          'gpio_slowdown', 'rp1_rio', 'scan_mode', 'disable_hardware_pulsing', 'inverse_colors', 'show_refresh_rate',
                          'pwm_bits', 'pwm_dither_bits', 'pwm_lsb_nanoseconds', 'limit_refresh_rate_hz', 'use_short_date_format',
                          'max_dynamic_duration_seconds', 'led_rgb_sequence', 'multiplexing', 'panel_type',
-                         'row_address_type', 'pixel_mapper_config']
+                         'row_address_type', 'pixel_mapper_config', 'orientation']
 
         if any(k in data for k in display_fields):
             if 'display' not in current_config:
@@ -831,6 +831,11 @@ def save_main_config():
             if 'pixel_mapper_config' in data and not isinstance(data['pixel_mapper_config'], str):
                 return jsonify({'status': 'error', 'message': 'pixel_mapper_config must be a string (e.g. "U-mapper;Rotate:90" or empty)'}), 400
 
+            # Validate orientation (physical mounting rotation; composed onto pixel_mapper_config at runtime)
+            ORIENTATION_ALLOWED = {'normal', '180'}
+            if 'orientation' in data and data['orientation'] not in ORIENTATION_ALLOWED:
+                return jsonify({'status': 'error', 'message': f"Invalid orientation '{data['orientation']}'. Allowed values: {', '.join(sorted(ORIENTATION_ALLOWED))}"}), 400
+
             # Validate row_address_type
             if 'row_address_type' in data:
                 try:
@@ -844,7 +849,7 @@ def save_main_config():
             for field in ['rows', 'cols', 'chain_length', 'parallel', 'brightness', 'hardware_mapping', 'scan_mode',
                          'pwm_bits', 'pwm_dither_bits', 'pwm_lsb_nanoseconds', 'limit_refresh_rate_hz',
                          'led_rgb_sequence', 'multiplexing', 'panel_type', 'row_address_type',
-                         'pixel_mapper_config']:
+                         'pixel_mapper_config', 'orientation']:
                 if field in data:
                     if field in ['rows', 'cols', 'chain_length', 'parallel', 'brightness', 'scan_mode',
                                'pwm_bits', 'pwm_dither_bits', 'pwm_lsb_nanoseconds', 'limit_refresh_rate_hz',