fix: correct scroll positioning and spinner layout in chat plan area

- Calculate step position relative to scroll container for accurate centering
- Add flex-shrink: 0 to spinner to prevent unwanted shrinking
- Pass alternateBackground prop to active step spinner

Components/ChatPlanArea.svelte

@@ -96,7 +96,8 @@
         const stepElement = stepElements[activeStepIndex];
         if (!stepElement) return;
 
-        const stepTop = stepElement.offsetTop;
+        // Calculate position relative to the scroll container (wrapperDiv)
+        const stepTop = stepElement.offsetTop - contentDiv.offsetTop;
         const stepHeight = stepElement.offsetHeight;
         const wrapperHeight = wrapperDiv.clientHeight;
         const scrollTo = stepTop - (wrapperHeight / 2) + (stepHeight / 2);
@@ -135,7 +136,7 @@
                         {/if}
                         {#if index === activeStepIndex}
                             <div class="chat-plan-step-icon">
-                                <Spinner {editModeActive}/>
+                                <Spinner {editModeActive} alternateBackground={true}/>
                             </div>
                         {/if}
                         {#if index > activeStepIndex}

Components/Spinner.svelte

@@ -31,6 +31,7 @@
     justify-content: center;
     align-items: center;
     border-radius: 50%;
+    flex-shrink: 0;
     --color: var(--interactive-accent);
     background: var(--color);
     background: linear-gradient(