fix(pdfx): guard PdfViewPinch._needReLayout against setState after dispose

[Yusuke-Shimamura]

What

_PdfViewPinchState._needReLayout schedules setState through Future.delayed(Duration.zero, ...) without checking mounted. When the viewer is unmounted in the same frame that scheduled the relayout (for example, the page hosting PdfViewPinch is popped while a jumpToPage / animateToPage tick or a relayout changed the set of visible pages), the callback lands on a disposed State.

• In debug builds this is setState() called after dispose().
• In release builds the assert is stripped and it surfaces as Null check operator used on a null value thrown from State.setState (_element!.markNeedsBuild()), which ends up in the app's zone error handler.

Observed in production on Flutter web with pdfx 2.9.2. Decoded stack:

State.setState                                          flutter/lib/src/widgets/framework.dart:1219
_PdfViewPinchState._needReLayout.<anonymous function>   pdfx-2.9.2/lib/src/viewer/pinch/pdf_view_pinch.dart:345
Future.delayed.<anonymous function>                     dart-sdk/lib/async/future.dart:440

Fix

Check mounted inside the delayed callback, matching the other delayed callbacks in this file (the loading-state listener and the first-attach callback in _reLayout already do this).

Notes

_updateRealSizeOverlay (#598) is a related but different path (async work continuing after dispose) and is not touched here.

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