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>

README.md

@@ -600,6 +600,14 @@ These settings are typically only needed for non-standard panels or custom confi
   - Leave empty unless you need custom mapping
   - See rpi-rgb-led-matrix documentation for full options
 
+- **`orientation`** (string, default: "normal")
+  - Rotates the rendered image to match how the panel is physically mounted
+  - Set to `"180"` (or use the "Upside Down" option in the web UI's Display
+    settings) if the panel is mounted upside down — useful for optimizing
+    where the Raspberry Pi and wiring sit relative to the mounting location
+  - Applied independently of `pixel_mapper_config` (appended as a trailing
+    `Rotate:180` mapper), so custom mapper configs keep working alongside it
+
 - **`row_address_type`** (integer, default: 0)
   - How rows are addressed on the panel
   - Most panels use 0 (direct addressing)