feat(invite): download the room QR code as a PNG

The invite QR was display-only. Add a "Download QR" button that exports it as a
PNG via an offscreen high-resolution (1024px, spec 4-module quiet zone)
QRCodeCanvas + canvas.toBlob, saved through useSaveFile (filename from the room
name) with the standard download toast. The visible code stays an SVG so it
renders crisply at any size/theme.

Also corrects the stale LOTUS_FEATURES note (the QR is generated locally via
qrcode.react, not api.qrserver.com).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

LOTUS_FEATURES.md

@@ -1102,7 +1102,8 @@ Persists via the `homeRoomSort` setting.
 
 `RoomShareInvite.tsx` provides a shareable invite UI:
 
-- 160×160px QR code generated via `api.qrserver.com`
+- 160×160px QR code generated locally via `qrcode.react` (`QRCodeSVG`) — no third-party service, works offline and under strict CSP, on a white quiet-zone so it scans on any theme
+- **Download QR**: exports the code as a PNG via an offscreen high-resolution (1024px, spec 4-module margin) `QRCodeCanvas` + `canvas.toBlob`, saved through `useSaveFile` (filename derived from the room name) with the standard download toast
 - "Copy Link" button to copy the `matrix.to` URI
 - Also accessible via a toggle button (⊞) in the Invite modal
 

src/app/features/common-settings/general/RoomShareInvite.tsx

@@ -1,6 +1,6 @@
-import React, { useCallback, useState } from 'react';
+import React, { useCallback, useRef, useState } from 'react';
 import { Box, Button, config, Icon, Icons, Text } from 'folds';
-import { QRCodeSVG } from 'qrcode.react';
+import { QRCodeSVG, QRCodeCanvas } from 'qrcode.react';
 import { SequenceCard } from '../../../components/sequence-card';
 import { SequenceCardStyle } from '../../room-settings/styles.css';
 import { SettingTile } from '../../../components/setting-tile';
@@ -8,11 +8,22 @@ import { CutoutCard } from '../../../components/cutout-card';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
 import { useRoom } from '../../../hooks/useRoom';
 import { getMatrixToRoom } from '../../../plugins/matrix-to';
+import { useSaveFile } from '../../../hooks/useSaveFile';
+
+// Filesystem-safe basename from an arbitrary room name.
+function sanitizeFilename(name: string): string {
+  const cleaned = name.replace(/[^a-z0-9\-_ ]+/gi, '').trim().replace(/\s+/g, '-');
+  return cleaned || 'room';
+}
 
 export function RoomShareInvite() {
   const mx = useMatrixClient();
   const room = useRoom();
+  const saveFile = useSaveFile();
   const [copied, setCopied] = useState(false);
+  // Offscreen high-resolution canvas used only to export the QR as a PNG; the
+  // visible code stays an SVG so it renders crisply at any size/theme.
+  const qrCanvasRef = useRef<HTMLCanvasElement>(null);
 
   const domain = mx.getDomain() ?? undefined;
   const inviteUrl = getMatrixToRoom(room.roomId, domain ? [domain] : undefined);
@@ -24,6 +35,14 @@ export function RoomShareInvite() {
     });
   }, [inviteUrl]);
 
+  const handleDownloadQr = useCallback(() => {
+    const canvas = qrCanvasRef.current;
+    if (!canvas) return;
+    canvas.toBlob((blob) => {
+      if (blob) saveFile(blob, `${sanitizeFilename(room.name)}-invite-qr.png`);
+    }, 'image/png');
+  }, [saveFile, room.name]);
+
   return (
     <SequenceCard
       className={SequenceCardStyle}
@@ -62,7 +81,7 @@ export function RoomShareInvite() {
               </Button>
             </Box>
           </Box>
-          <Box justifyContent="Center">
+          <Box direction="Column" alignItems="Center" gap="300">
             {/* Generated locally (qrcode.react) — no third-party service, works
                 offline + under strict CSP. White padded quiet-zone so the
                 default black-on-white code scans on any theme. */}
@@ -76,9 +95,28 @@ export function RoomShareInvite() {
             >
               <QRCodeSVG value={inviteUrl} size={160} level="M" title="Room invite QR code" />
             </Box>
+            <Button
+              size="300"
+              variant="Secondary"
+              fill="Soft"
+              radii="300"
+              onClick={handleDownloadQr}
+              before={<Icon size="100" src={Icons.Download} />}
+            >
+              <Text size="B300">Download QR</Text>
+            </Button>
           </Box>
         </Box>
       </CutoutCard>
+      {/* Offscreen, high-res (spec 4-module quiet zone) source for PNG export. */}
+      <QRCodeCanvas
+        ref={qrCanvasRef}
+        value={inviteUrl}
+        size={1024}
+        level="M"
+        marginSize={4}
+        style={{ display: 'none' }}
+      />
     </SequenceCard>
   );
 }