fix(articles): snapshot scroller ref in HighlightLayer effect teardown

Back-navigating from an article detail view to the list and into the
same article again crashed with
  TypeError: Cannot read properties of null (reading 'removeEventListener')

Sequence that triggered it:
 1. ReaderView unmounts, its own $effect cleanup calls onscroller(null).
 2. DetailView sets readerScroller = null.
 3. HighlightLayer's prop `scroller` becomes null.
 4. The old $effect's teardown fires and reads `scroller` — which now
    points at null instead of the element it had attached listeners to.
 5. null.removeEventListener(...) throws, Svelte can't finish tearing
    down the tree, and the re-mount never happens.

Fix: snapshot the element reference at setup time so the teardown uses
the same element the setup used, regardless of what the reactive prop
is currently pointing at. Comment block in the file explains the trap
so a future cleanup doesn't re-introduce it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/mana/apps/web/src/lib/modules/articles/components/HighlightLayer.svelte

@@ -154,13 +154,22 @@
 	}
 
 	$effect(() => {
-		if (!scroller) return;
-		scroller.addEventListener('mouseup', onSelectionEnd);
-		scroller.addEventListener('click', onClick);
+		// Snapshot the element ref at setup time. `scroller` is a reactive
+		// prop: when the parent navigates away and re-mounts the Reader,
+		// it first pushes `scroller = null`, then `scroller = newEl`.
+		// Reading `scroller` inside the teardown returned below would
+		// observe whichever value is live *at teardown*, not the one we
+		// attached listeners to — which caused
+		// "Cannot read properties of null (reading 'removeEventListener')"
+		// on back-navigation between two article detail views.
+		const el = scroller;
+		if (!el) return;
+		el.addEventListener('mouseup', onSelectionEnd);
+		el.addEventListener('click', onClick);
 		document.addEventListener('mousedown', onMousedown);
 		return () => {
-			scroller.removeEventListener('mouseup', onSelectionEnd);
-			scroller.removeEventListener('click', onClick);
+			el.removeEventListener('mouseup', onSelectionEnd);
+			el.removeEventListener('click', onClick);
 			document.removeEventListener('mousedown', onMousedown);
 		};
 	});