Fix crash loop: WebView touched off main thread during extraction cancellation

Root-caused via emulator stress test (rapid swiping through many articles):
withTimeoutOrNull's cancellation, and LaunchedEffect disposal when a page is
swiped away, both invoke invokeOnCancellation on kotlinx.coroutines.DefaultExecutor
rather than the calling coroutine's Main dispatcher. The handler called
webView.stopLoading()/destroy() directly, crashing with "A WebView method was
called on thread ..." on essentially every session once enough thin-content
articles were swiped past mid-extraction -- the reported crash loop. Now hops
back to the main looper before touching the WebView, and guards against a
double destroy() race between the JS-callback and cancellation paths.

Also fixed a real (non-crashing) leak: the visible reader WebView in
ReaderScreen was never destroy()-ed when its page left the pager, via
AndroidView's onRelease.

Reproduced pre-fix in ~80 rapid swipes on an emulator; post-fix, 320+ swipes
with real 107-feed data ran clean with stable native heap.

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