feat: replace completion animation with Gemini-style rotating gradient glow

- Remove bouncing "CRISPY" text animation (dvd-bounce)
- Add rotating conic-gradient border with glow effect
- Colors: purple (#9b59b6), magenta (#e040fb), cyan (#00bcd4), violet (#7c4dff)
- Animation plays once (2.5s) then fades out so students can continue
- Update success colors from green to purple theme:
  - success-color: #9b6dd4
  - success-color-dark: #7c4dff
  - success-color-light: #c9b8e8
- Uses CSS @property for animating conic-gradient angle
- Two layers: blurred glow + crisp border using mask-composite

🤖 Generated with [Claude Code](https://claude.com/claude-code)

src/app.js

@@ -626,11 +626,11 @@ function runCode() {
 		elements.nextBtn.classList.add("success");
 		elements.taskInstruction.classList.add("success-instruction");
 
-		// Show match animation (DVD-style bouncing)
+		// Show match animation (rotating gradient glow)
 		elements.previewWrapper?.classList.add("matched");
 		setTimeout(() => {
 			elements.previewWrapper?.classList.remove("matched");
-		}, 10000);
+		}, 3000);
 
 		updateNavigationButtons();
 		updateProgressDisplay();