feat: desaturate quest markers for trivial (gray) quests

Trivial/low-level quests now show gray '!' / '?' markers instead of
yellow, matching the in-game distinction between available and trivial
quests. Add grayscale parameter to QuestMarkerRenderer::setMarker and
the push-constant block; application sets grayscale=1.0 for trivial
markers and 0.0 for all others.

assets/shaders/quest_marker.frag.glsl

@@ -5,6 +5,7 @@ layout(set = 1, binding = 0) uniform sampler2D markerTexture;
 layout(push_constant) uniform Push {
     mat4 model;
     float alpha;
+    float grayscale;  // 0 = full colour, 1 = fully desaturated (trivial quests)
 } push;
 
 layout(location = 0) in vec2 TexCoord;
@@ -14,5 +15,7 @@ layout(location = 0) out vec4 outColor;
 void main() {
     vec4 texColor = texture(markerTexture, TexCoord);
     if (texColor.a < 0.1) discard;
-    outColor = vec4(texColor.rgb, texColor.a * push.alpha);
+    float lum = dot(texColor.rgb, vec3(0.299, 0.587, 0.114));
+    vec3 rgb = mix(texColor.rgb, vec3(lum), push.grayscale);
+    outColor = vec4(rgb, texColor.a * push.alpha);
 }