fix(file-viewer): keep rendering a PDF after a file-watcher reload

An open PDF went blank the moment the watcher reported the file had
changed, and stayed blank for the rest of the session — every later
version of the file too.

<pdf-view>._teardown() released the previous document with
PDFDocumentProxy.destroy(), a method pdf.js no longer has: a document is
torn down through its loading task. The absent method threw a TypeError,
and _teardown() is the *first* statement of _open(): the page list had
already been emptied, so nothing after the throw ran — no new document
was loaded, and the emptied .pdfv-pages had nothing to refill it. Since
_doc was never cleared either, every subsequent src hit the same throw,
which is why the viewer never recovered. _open() is async and its caller
(updated()) does not await it, so the TypeError surfaced only as an
unhandled rejection.

Tear the document down through doc.loadingTask.destroy() instead, and
swallow its failure: releasing the previous document must never be able
to stop the next one from loading. The same call in _open()'s
stale-document path had the identical bug.

Verified in headless Chromium against the real component: swapping the
blob URL the way FileViewerBase._load does now reloads the document
(5 pages -> 7 -> 5, correct text layer, no exceptions), including three
reloads fired back-to-back so the stale-document path is exercised.

web/components/shared/pdf-view.js

@@ -62,6 +62,23 @@ const MAX_DPR = 2;
 
 let _pdfjsPromise = null;
 
+/**
+ * Tear a document down. pdf.js dropped `PDFDocumentProxy.destroy()` — the
+ * document is released through its loading task — and calling the absent method
+ * threw a TypeError out of `_teardown()`, which is called *first* in `_open()`:
+ * the page list had already been emptied, the rest of `_open()` never ran, and
+ * the viewer stayed blank for the rest of its life (every later src hit the same
+ * throw). That is what a file-watcher reload of an open PDF looked like. Hence
+ * also the swallow: releasing the previous document must never be able to stop
+ * the next one from loading.
+ */
+function destroyDoc(doc) {
+  try {
+    const p = doc?.loadingTask?.destroy();
+    if (p?.catch) p.catch(() => { /* already gone */ });
+  } catch { /* already gone */ }
+}
+
 /** Import pdf.js once per page load and point it at the vendored worker. */
 function loadPdfjs() {
   if (!_pdfjsPromise) {
@@ -131,7 +148,7 @@ export class PdfView extends LightElement {
         standardFontDataUrl: STD_FONTS_URL,
       }).promise;
       // A newer src landed while we were loading — drop this one on the floor.
-      if (seq !== this._loadSeq) { doc.destroy(); return; }
+      if (seq !== this._loadSeq) { destroyDoc(doc); return; }
       this._pdfjs = pdfjs;
       this._doc   = doc;
       this._total = doc.numPages;
@@ -186,8 +203,9 @@ export class PdfView extends LightElement {
     for (const slot of this._slots) this._release(slot);
     this._slots = [];
     this.querySelector('.pdfv-pages')?.replaceChildren();
-    this._doc?.destroy();
+    const doc = this._doc;
     this._doc = null;
+    destroyDoc(doc);
     this._lastWidth = 0;
   }