fix: simplify PPT preview zoom to use direct scaling

Replace the inverse-scale technique (width: 100/scale%, transform: scale())
with direct scaling (width: 100%, transform: scale()). The previous approach
caused the container size to shrink as zoom increased, which could affect
pagination controls and other overlay elements that depend on container
dimensions.

The new approach:
- Sets container to 100% width/height
- Applies transform: scale() directly
- Keeps zoom direction intuitive (+ enlarges, - shrinks)
- Prevents pagination controls from scaling with content

Fixes #3177

apps/web/src/components/FileViewer.tsx

@@ -516,8 +516,8 @@ function previewScaleShellStyle(
 ): CSSProperties & Record<string, string | number> {
   if (viewport === 'desktop') {
     return {
-      width: `${100 / previewScale}%`,
-      height: `${100 / previewScale}%`,
+      width: '100%',
+      height: '100%',
       transform: `scale(${previewScale})`,
       transformOrigin: '0 0',
     };
@@ -537,8 +537,8 @@ function manualEditPreviewShellStyle(
 ): CSSProperties & Record<string, string | number> {
   if (viewport === 'desktop' && frozenWidth) {
     return {
-      width: `${frozenWidth / previewScale}px`,
-      height: `${100 / previewScale}%`,
+      width: `${frozenWidth}px`,
+      height: '100%',
       transform: `scale(${previewScale})`,
       transformOrigin: '0 0',
     };