Let each visibility switch hide only its own thing

The margin notes hung off both switches: `hidden: hideInk || !showPins`, so
turning off the drawing took the markers and their column with it — one switch
turning off both of the things the pair exists to separate.

The checks look at what is actually visible — the ink layer's opacity, how many
markers are on the page, whether the column is there — and test both directions
separately, which is what the old code failed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/features/pdf/PdfViewer.jsx

@@ -804,7 +804,11 @@ export function PdfViewer({ note, dark, onUpdate, onPagesReady, notesToggle, foc
                   notes: notesForPage(ink, i + 1),
                   placing,
                   openId: openPin,
-                  hidden: hideInk || !showPins,
+                  // Only the margin-note switch decides this. It used to read
+                  // `hideInk || !showPins`, so hiding the drawing took the
+                  // markers and their column with it — one switch turning off
+                  // both things the pair was meant to separate.
+                  hidden: !showPins,
                   withMargin,
                   onOpen: setOpenPin,
                   onPlace: placeNote,