fix(web): reset image and PDF preview when the modal switches documents

DocumentContent renders ImagePreview and PdfViewer without a key, so React
keeps the same instance alive when the open document modal moves to another
document. Both components seed internal state from their props on mount and
never resync:

- ImagePreview stores the source in `activeSrc = useState(url)`, so opening a
  second image keeps showing the first one (and a prior load error keeps the
  "Failed to load image" state).
- PdfViewer holds `cachedUrl` from the previous document. Its effect revokes
  the old object URL when documentId changes, but the state still points at the
  now-revoked blob URL, and fileSource returns it first, so the next PDF fails
  to render.

Key both previews by document id so switching documents mounts a fresh
instance with clean state, and the existing unmount cleanup revokes the old
object URLs.

apps/web/components/document-modal/content/index.tsx

@@ -90,6 +90,7 @@ export function DocumentContent({
 		case "image":
 			return (
 				<ImagePreview
+					key={document.id}
 					url={document.url ?? ""}
 					title={document.title}
 					documentId={document.id}
@@ -109,7 +110,13 @@ export function DocumentContent({
 			return <TextEditorContent {...textEditorProps} />
 
 		case "pdf":
-			return <PdfViewer url={document.url} documentId={document.id} />
+			return (
+				<PdfViewer
+					key={document.id}
+					url={document.url}
+					documentId={document.id}
+				/>
+			)
 
 		case "notion":
 			return <NotionDoc content={document.content ?? ""} />