Sort a notebook, import Markdown back, and stop the page shrinking in Draw

The paper measurably shrank on switching to Draw — 733px to 460px on the same
screen. The editor was a flex item at its default `flex: 0 1 auto`, so its width
was its widest row's: the toolbar. Swapping a dozen formatting buttons for six
drawing ones collapsed it. It now fills the pane, so the width is a property of
the window.

Each notebook remembers how its notes are ordered, on the server rather than per
device. The manual order has to survive being looked away from, so `reindex`
renumbers every note in the notebook rather than the filtered view, and dragging
is switched off while a search narrows the list. Reordering has its own route:
`PUT /notes/:id` would bump `updated_at` and snapshot a version, so a drag would
move a note to the top of "last changed" and spend a slot of its history.

Markdown and our own ZIP archives come back as notes, with their images
re-uploaded as attachments. A drawing returns as the PNG it was exported as —
vectors are not recoverable from pixels — and the import says so rather than
leaving it to be discovered. Writing the parser turned up a real bug in the
exporter: blocks were emitted with no blank line between them, so a list after a
list became one list and a quote after an item was read as part of it.

Creating or importing from "All Notes" now asks which notebook instead of
silently taking the first one. The web-link prompt is a real dialog: a bare
domain becomes https, javascript: is refused with a reason. The PDF viewer's
hide-drawing control is a labelled button rather than a filled-or-hollow dot,
and the export icon points down into a tray like every other download.

186 unit tests, 179 browser checks across eleven suites, including a new
`offline` suite that loads the app cold with the network off.

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

frontend/src/features/pdf/PdfViewer.jsx

@@ -464,14 +464,32 @@ export function PdfViewer({ note, dark, onUpdate, onPagesReady }) {
           </>
         )}
 
-        <button
-          data-pdf-hide-ink={hideInk ? "on" : "off"}
-          onClick={() => setHideInk((v) => !v)}
-          title={hideInk ? "Zeichnung einblenden" : "Zeichnung ausblenden"}
-          style={{ ...barButton, color: hideInk ? T.accent : T.muted }}
-        >
-          {hideInk ? "◌" : "●"}
-        </button>
+        {/* A labelled control, not a dot. Hiding the ink changes what the page
+            shows, so it has to be findable without being clicked to find out —
+            and its state has to be readable at a glance rather than from the
+            difference between a filled and a hollow circle.
+            Only shown once there is a drawing: until then it would offer to hide
+            nothing. */}
+        {totalStrokes(ink) > 0 && (
+          <button
+            data-pdf-hide-ink={hideInk ? "on" : "off"}
+            onClick={() => setHideInk((v) => !v)}
+            title={hideInk ? "Zeichnung wieder einblenden" : "Zeichnung ausblenden"}
+            style={{
+              ...barButton,
+              width: "auto",
+              padding: "0 10px",
+              gap: 6,
+              border: `1px solid ${hideInk ? T.accent + "66" : T.border}`,
+              background: hideInk ? T.accent + "1a" : "transparent",
+              color: hideInk ? T.accent : T.muted,
+              fontWeight: 600,
+              fontSize: 12,
+            }}
+          >
+            {hideInk ? I.eyeOff : I.eye} {hideInk ? "Zeichnung aus" : "Zeichnung an"}
+          </button>
+        )}
 
         <div style={{ marginLeft: "auto", display: "flex", alignItems: "center", gap: 6 }}>
           <button onClick={() => stepZoom(-1)} title="Kleiner" style={barButton}>