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/texts.json

@@ -0,0 +1,17 @@
+{
+  "top": [
+    {
+      "text": "Witamy w PTI",
+      "colors": [
+        {"r": 40, "g": 40, "b": 40, "from": 0},
+        {"r": 0,  "g": 0,  "b": 50, "from": 9}
+      ]
+    }
+  ],
+  "bottom": [
+    {"text": "Technik informatyk + ai, robotyka",          "colors": [{"r": 0,  "g": 0,  "b": 50, "from": 0}]},
+    {"text": "Technik programista",                         "colors": [{"r": 50, "g": 40, "b": 0,  "from": 0}]},
+    {"text": "Technik poligrafii i grafiki komputerowej",  "colors": [{"r": 0,  "g": 45, "b": 0,  "from": 0}]},
+    {"text": "Technik reklamy",                            "colors": [{"r": 60, "g": 20, "b": 0,  "from": 0}]}
+  ]
+}