Update README and code to set default sparkle shape to 'none' and add opacity control

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

README.md

@@ -13,8 +13,9 @@ It exposes two widget layers:
 - Optional motion input via sensors or custom tilt stream
 - Generic wrapper API for any widget via `Shiny(child: ...)`
 - Card API with `background`, `foreground`, `shape`, and drag tilt via `ShinyCard`
-- Built-in sparkle presets including 8-point star, 5-point star, rectangle, diamond, hexagon, random polygon, and confetti
+- Built-in sparkle presets including `none` (default), 8-point star, 5-point star, rectangle, diamond, hexagon, random polygon, and confetti
 - Custom sparkle shapes via parameterized `SparkleShapeSpec` factories
+- Global shader opacity control via `opacity`
 - Cross-platform Flutter support (mobile, web, desktop)
 
 ## Installation
@@ -60,7 +61,8 @@ ShinyCard(
   controller: controller,
   background: Container(color: const Color(0xFF1B2D4B)),
   foreground: const Center(child: Text('HOLO')),
-  sparkleShape: SparkleShapeSpec.eightPointStar,
+  sparkleShape: SparkleShapeSpec.none,
+  opacity: 1.0,
 )
 ```
 
@@ -80,6 +82,10 @@ Shiny(
 Use built-in sparkle presets:
 
 ```dart
+ShinyCard(
+  sparkleShape: SparkleShapeSpec.none,
+)
+
 ShinyCard(
   sparkleShape: SparkleShapeSpec.hexagon,
 )
@@ -120,6 +126,11 @@ final ShinyController controller = ShinyController(tiltStream: input.stream);
 - `ShinyController`: optional source selection for tilt
 - `SensorTiltController`: low-level sensor fusion stream utility
 
+Important defaults:
+
+- `sparkleShape` defaults to `SparkleShapeSpec.none` (no sparkles)
+- `opacity` defaults to `1.0`
+
 ## Platform Notes
 
 - Shader uses Flutter runtime effects and avoids derivative functions (`dFdx`, `dFdy`, `fwidth`) for web compatibility.