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>

config/config.template.json

@@ -112,6 +112,7 @@
             "led_rgb_sequence": "RGB",
             "limit_refresh_rate_hz": 100,
             "pixel_mapper_config": "",
+            "orientation": "normal",
             "row_address_type": 0,
             "multiplexing": 0,
             "panel_type": ""