add mock mode, websocket display stream, and text editor

- mock_rpi_ws281x: stub for running without RPi hardware; auto-fallback
  on RuntimeError, forced via --mock flag or MOCK_LED env var
- mock mode runs web server on port 8080 instead of 80
- replace display polling with WebSocket stream (/ws/display); client
  sends start/stop messages, server pushes binary RGB frames
- force-button feedback: button shows waiting state and polls until
  the cycle clears the flag
- text editor UI: edit top and bottom texts with per-segment color
  pickers; saved to texts.json for persistence across restarts
- bottom and top texts read from server state instead of hardcoded
- bottom text switched to _add_multi_color_node to support multi-color
- fix bottom text invisible: split scroll_all_multi_color_texts into
  top (y<8) and bottom (y>=8) passes so fill_pixels does not wipe
  bottom nodes before they are drawn; this also fixes force-vehicles
  polling loop that stalled when the bottom node never expired

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

rpi/low_level.py

@@ -1,4 +1,7 @@
-from rpi_ws281x import Color
+try:
+    from rpi_ws281x import Color
+except (ImportError, RuntimeError):
+    from mock_rpi_ws281x import Color
 from config import PANEL_PIXEL_COUNT, DISPLAY_MAX_X, DISPLAY_MAX_Y
 from fonts import FONT_7X5