fix(effects): stop retrying scroll-fade shader init after it fails

If the scroll-edge fade shader failed to compile or link, the init path
left its ready flag clear and tried again on the very next frame. On a
driver that rejects the shader that meant a fresh compile attempt every
frame, for the life of the program.

- A failed init is now latched and treated as final until destroy(), so
  the compile is attempted once, the failure is logged once, and the
  fades stay disabled without further retries. Destroying the shader
  clears the latch so a later re-init can try again.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/effects/scroll_fade_shader.h

@@ -49,6 +49,7 @@ struct ScrollFadeShader {
 
     // ---- Internal state ----
     bool ready = false;
+    bool failed = false;   // a failed init is final until destroy(): no per-frame recompiles on a rejecting driver
 
 #ifdef DRAGONX_HAS_GLAD
     GLuint program_       = 0;
@@ -72,11 +73,16 @@ struct ScrollFadeShader {
 
     bool init() {
         if (ready) return true;
+        if (failed) return false;
 #ifdef DRAGONX_HAS_GLAD
         ready = initGL();
 #elif defined(DRAGONX_USE_DX11)
         ready = initDX11();
 #endif
+        if (!ready) {
+            failed = true;
+            LOGF("ScrollFadeShader: init failed, scroll-edge fades disabled\n");   // once; release builds record it
+        }
         return ready;
     }
 
@@ -87,6 +93,7 @@ struct ScrollFadeShader {
         destroyDX11();
 #endif
         ready = false;
+        failed = false;
     }
 
     // ----------------------------------------------------------------