fix(ui): update icons and styles for step notices and modal

Replace exclamation-circle with a simpler exclamation icon and add
top margin in the previous steps incomplete notice to improve alignment.
Increase size and change color from green to teal on the check-circle
icon in the tests passed notice for better visibility.
Swap emoji warning in the incomplete modal with a consistent SVG icon
to maintain visual consistency and improve styling control.

Author: rohitpaulk

app/components/course-page/course-stage-step/tests-passed-notice.hbs

@@ -6,7 +6,7 @@
 >
   <div class="flex items-center flex-wrap gap-4">
     <div class="flex items-center gap-2">
-      {{svg-jar "check-circle" class="w-6 h-6 fill-green-500"}}
+      {{svg-jar "check-circle" class="w-8 h-8 fill-teal-500"}}
 
       <div class="prose prose-sm prose-green dark:prose-invert">
         <p>All tests passed!</p>

app/components/course-page/previous-steps-incomplete-modal.hbs

@@ -1,8 +1,6 @@
 <ModalBody @allowManualClose={{false}} @onClose={{@onClose}} data-test-previous-steps-incomplete-modal class="max-w-xl" ...attributes>
   <div class="flex flex-col items-center w-full max-w-2xs">
-    <div class="text-[60px] text-center">
-      ⚠️
-    </div>
+    {{svg-jar "exclamation" class="w-16 h-16 mb-3 fill-yellow-500"}}
 
     <div class="mb-3 font-bold text-2xl text-gray-700 dark:text-gray-100 text-center">
       Previous steps incomplete!

app/components/course-page/previous-steps-incomplete-notice.hbs

@@ -5,7 +5,7 @@
 >
   <div class="flex items-center flex-wrap gap-4">
     <div class="flex items-center gap-2">
-      {{svg-jar "exclamation-circle" class="w-8 h-8 fill-yellow-500"}}
+      {{svg-jar "exclamation" class="w-8 h-8 fill-yellow-500 mt-1"}}
 
       <div class="prose prose-sm prose-yellow dark:prose-invert">
         <p>Complete previous steps to access this stage.</p>