feat(shadertoy): add proper mouse/touch interaction support (#293)

* feat(shadertoy): add proper mouse/touch interaction support

- Fix iMouse uniform to match official Shadertoy specification:
  - iMouse.xy only updates while button/touch is held down
  - iMouse.zw uses positive values when down, negative when released
  - Enables proper click detection via iMouse.z > 0

- Add touch event support for mobile devices:
  - touchstart/touchmove/touchend/touchcancel handlers
  - Same behavior as mouse events

- Update tool description with mouse interaction documentation
- Update README with:
  - Mouse & Touch Interaction feature
  - Improved Julia Set example with click detection
  - iMouse specification table and usage examples

The iMouse uniform is a core Shadertoy feature used by popular shaders
like Star Nest, raymarching examples, and particle systems.

* style: fix Prettier formatting in shadertoy README

examples/shadertoy-server/server.ts

@@ -33,10 +33,24 @@ AVAILABLE UNIFORMS:
 - iTime (float): elapsed time in seconds
 - iTimeDelta (float): time since last frame
 - iFrame (int): frame counter
-- iMouse (vec4): mouse position (xy=current, zw=click)
+- iMouse (vec4): mouse/touch position in pixels (see MOUSE INTERACTION below)
 - iDate (vec4): year, month, day, seconds
 - iChannel0-3 (sampler2D): buffer inputs for multi-pass shaders
 
+MOUSE INTERACTION:
+The iMouse uniform provides interactive mouse/touch input (works on mobile):
+- iMouse.xy: Current position while button/touch is held down (frozen on release)
+- iMouse.zw: Click start position (positive when down, negative when released)
+- iMouse.z > 0: Button is currently pressed
+- iMouse.z < 0: Button was released
+- iMouse.z == 0: Never clicked
+
+Example - camera control:
+  vec2 uv = iMouse.xy / iResolution.xy;  // normalized 0-1
+
+Example - detect click:
+  if (iMouse.z > 0.0) { /* button is down */ }
+
 MULTI-PASS RENDERING:
 - Use bufferA-D parameters for feedback effects, blur chains, simulations
 - BufferA output -> iChannel0, BufferB -> iChannel1, etc.