Implement Text Input

web/assets/backgrounds/image_spec.md

@@ -45,6 +45,7 @@ This file tells the web UI where your letters/targets are located in **physical
 
 ```json
 {
+  "hasAlphabet": true,
   "bounds": {
     "xMin": -150,
     "xMax": 150,
@@ -58,6 +59,7 @@ This file tells the web UI where your letters/targets are located in **physical
   ]
 }
 ```
+- **hasAlphabet**: (Optional, boolean) If `true`, this background is used for the "Input Text" feature. The letters in `spots` are used to spell out typed input.
 - **bounds**: Defines the physical bounding box (in mm) that your background image represents. This automatically adjusts the UI's aspect ratio and coordinate mapping!
 - **spots**: Array of target objects.
 - **id**: Unique identifier for the dropdown list.