fix(viewer): make initial zoom setup robust and clear timers in ZoomAPIBridge (#4588)

# Description of Changes

**What was changed**
- Reworked the initial zoom `useEffect`:
- Early return when `zoom` is unavailable or initial zoom was already
applied.
  - Extracted an `attemptInitialZoom` function with a single retry path.
- Introduced proper timeout handles (`timer`, `retryTimer`) and added a
cleanup function to clear them on unmount/re-render.
- Expanded the effect dependency array to include `zoomState` to avoid
stale state issues.
- Switched to nullish coalescing for safer defaulting of
`currentZoomLevel` (`zoomState.currentZoomLevel ?? 1.4`).
- Minor logging adjustments to clarify delayed/failed initialization
paths.

**Why the change was made**
- The previous implementation risked leaving dangling timers and could
re-attempt zoom unnecessarily, causing flicker or inconsistent initial
zoom when the viewport wasn’t ready.
- Including `zoomState` in dependencies ensures the component reacts to
state changes correctly and avoids stale reads.
- Cleanup of timers prevents memory leaks and race conditions during
rapid mounts/unmounts or navigation.

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Reece Browne <74901996+reecebrowne@users.noreply.github.com>

frontend/src/components/viewer/ZoomAPIBridge.tsx

@@ -12,30 +12,42 @@ export function ZoomAPIBridge() {
 
   // Set initial zoom once when plugin is ready
   useEffect(() => {
-    if (zoom && !hasSetInitialZoom.current) {
-      hasSetInitialZoom.current = true;
-      setTimeout(() => {
-        try {
-          zoom.requestZoom(1.4);
-        } catch (error) {
-          console.log('Zoom initialization delayed, viewport not ready:', error);
-          // Retry after a longer delay
-          setTimeout(() => {
-            try {
-              zoom.requestZoom(1.4);
-            } catch (retryError) {
-              console.log('Zoom initialization failed:', retryError);
-            }
-          }, 200);
-        }
-      }, 50);
+    if (!zoom || hasSetInitialZoom.current) {
+      return;
     }
-  }, [zoom]);
+
+    let retryTimer: ReturnType<typeof setTimeout> | undefined;
+    const attemptInitialZoom = () => {
+      try {
+        zoom.requestZoom(1.4);
+        hasSetInitialZoom.current = true;
+      } catch (error) {
+        console.log('Zoom initialization delayed, viewport not ready:', error);
+        retryTimer = setTimeout(() => {
+          try {
+            zoom.requestZoom(1.4);
+            hasSetInitialZoom.current = true;
+          } catch (retryError) {
+            console.log('Zoom initialization failed:', retryError);
+          }
+        }, 200);
+      }
+    };
+
+    const timer = setTimeout(attemptInitialZoom, 50);
+
+    return () => {
+      clearTimeout(timer);
+      if (retryTimer) {
+        clearTimeout(retryTimer);
+      }
+    };
+  }, [zoom, zoomState]);
 
   useEffect(() => {
     if (zoom && zoomState) {
       // Update local state
-      const currentZoomLevel = zoomState.currentZoomLevel || 1.4;
+      const currentZoomLevel = zoomState.currentZoomLevel ?? 1.4;
       const newState = {
         currentZoom: currentZoomLevel,
         zoomPercent: Math.round(currentZoomLevel * 100),