feat: explain "Failed to send" when the homeserver wants its terms accepted

A homeserver with a consent requirement (Synapse user_consent) rejects sends
with 403 M_CONSENT_NOT_GIVEN until the user accepts its current terms. The
message just showed "Failed to send" with no reason.

Listen for the SDK's HttpApiEvent.NoConsent and show a dialog naming the
user's own homeserver (the client works with any server, so no Lotus-specific
wording), with "Review and accept" opening the server's consent_uri (http(s)
only; anything else is dropped) and "I've accepted — retry sending" resending
every event that failed for this reason. "Later" snoozes it for 10 s so
background retries don't re-open it immediately.

Verified in Chromium against a local Synapse with the send endpoint answering
M_CONSENT_NOT_GIVEN: dialog shows, link opens, retry delivers the message.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/ConsentRequiredPrompt.tsx

@@ -0,0 +1,168 @@
+import React, { useCallback, useEffect, useRef, useState } from 'react';
+import FocusTrap from 'focus-trap-react';
+import {
+  Box,
+  Button,
+  Dialog,
+  Header,
+  Icon,
+  Icons,
+  Overlay,
+  OverlayBackdrop,
+  OverlayCenter,
+  Text,
+  config,
+} from 'folds';
+import {
+  EventStatus,
+  HttpApiEvent,
+  HttpApiEventHandlerMap,
+  MatrixEvent,
+  Room,
+  RoomEvent,
+  RoomEventHandlerMap,
+} from 'matrix-js-sdk';
+import { useMatrixClient } from '../hooks/useMatrixClient';
+import { useModalStyle } from '../hooks/useModalStyle';
+import { failedForConsent, safeConsentUri } from '../utils/consent';
+
+// After "Later", don't re-open for every background retry; the next send the
+// user makes after this window explains again.
+const SNOOZE_MS = 10_000;
+
+/**
+ * A homeserver with a consent requirement (e.g. Synapse's user_consent) blocks
+ * sending until its terms are accepted, answering 403 M_CONSENT_NOT_GIVEN with a `consent_uri`. Without
+ * this the message just shows "Failed to send". The SDK emits
+ * HttpApiEvent.NoConsent for every such response; we explain why, link to the
+ * acceptance page, and offer to resend what was blocked once accepted.
+ */
+export function ConsentRequiredPrompt() {
+  const mx = useMatrixClient();
+  const [consentUri, setConsentUri] = useState<string>();
+  const [open, setOpen] = useState(false);
+  const [opened, setOpened] = useState(false);
+  const [blockedCount, setBlockedCount] = useState(0);
+  const snoozedUntil = useRef(0);
+  const blocked = useRef(new Map<MatrixEvent, Room>());
+  const modalStyle = useModalStyle(440);
+  const server = mx.getDomain() ?? 'Your homeserver';
+
+  useEffect(() => {
+    const onNoConsent: HttpApiEventHandlerMap[HttpApiEvent.NoConsent] = (_message, uri) => {
+      const safe = safeConsentUri(uri);
+      if (safe) setConsentUri(safe);
+      if (Date.now() >= snoozedUntil.current) setOpen(true);
+    };
+    const onLocalEcho: RoomEventHandlerMap[RoomEvent.LocalEchoUpdated] = (event, room) => {
+      if (failedForConsent(event)) blocked.current.set(event, room);
+      else if (!blocked.current.delete(event)) return;
+      setBlockedCount(blocked.current.size);
+    };
+    mx.on(HttpApiEvent.NoConsent, onNoConsent);
+    mx.on(RoomEvent.LocalEchoUpdated, onLocalEcho);
+    return () => {
+      mx.removeListener(HttpApiEvent.NoConsent, onNoConsent);
+      mx.removeListener(RoomEvent.LocalEchoUpdated, onLocalEcho);
+    };
+  }, [mx]);
+
+  const review = useCallback(() => {
+    if (!consentUri) return;
+    window.open(consentUri, '_blank', 'noopener,noreferrer');
+    setOpened(true);
+  }, [consentUri]);
+
+  const retry = useCallback(() => {
+    setOpen(false);
+    setOpened(false);
+    const pending = Array.from(blocked.current.entries());
+    blocked.current.clear();
+    setBlockedCount(0);
+    pending.forEach(([event, room]) => {
+      if (event.status === EventStatus.NOT_SENT) mx.resendEvent(event, room);
+    });
+  }, [mx]);
+
+  const later = useCallback(() => {
+    snoozedUntil.current = Date.now() + SNOOZE_MS;
+    setOpen(false);
+  }, []);
+
+  if (!open) return null;
+  return (
+    <Overlay open backdrop={<OverlayBackdrop />}>
+      <OverlayCenter>
+        <FocusTrap
+          focusTrapOptions={{
+            initialFocus: '#consent-review',
+            fallbackFocus: '#consent-later',
+            onDeactivate: later,
+            clickOutsideDeactivates: true,
+            escapeDeactivates: true,
+          }}
+        >
+          <Dialog
+            variant="Surface"
+            role="alertdialog"
+            aria-modal="true"
+            aria-labelledby="consent-title"
+            aria-describedby="consent-body"
+            style={modalStyle}
+          >
+            <Header
+              style={{
+                padding: `0 ${config.space.S400}`,
+                borderBottomWidth: config.borderWidth.B300,
+              }}
+              variant="Surface"
+              size="500"
+            >
+              <Text as="h2" size="H4" id="consent-title">
+                Accept the terms to keep chatting
+              </Text>
+            </Header>
+            <Box style={{ padding: config.space.S400 }} direction="Column" gap="400">
+              <Box id="consent-body" direction="Column" gap="200">
+                <Text priority="400">
+                  Your message wasn&apos;t sent. <b>{server}</b> requires you to review and accept
+                  its terms of service before you can send messages.
+                </Text>
+                <Text priority="400">
+                  {consentUri
+                    ? 'Nothing you sent is lost: accept the terms, then come back here and retry.'
+                    : 'Your homeserver didn’t include a link to its terms. Contact its administrator, or accept them in another Matrix app, then retry.'}
+                </Text>
+              </Box>
+              <Box direction="Column" gap="200">
+                {consentUri && (
+                  <Button
+                    id="consent-review"
+                    variant={opened ? 'Secondary' : 'Primary'}
+                    fill={opened ? 'Soft' : 'Solid'}
+                    onClick={review}
+                    after={<Icon size="100" src={Icons.External} />}
+                  >
+                    <Text size="B400">Review and accept</Text>
+                  </Button>
+                )}
+                <Button
+                  variant={opened ? 'Primary' : 'Secondary'}
+                  fill={opened ? 'Solid' : 'Soft'}
+                  onClick={retry}
+                >
+                  <Text size="B400">
+                    {blockedCount > 0 ? "I've accepted — retry sending" : "I've accepted"}
+                  </Text>
+                </Button>
+                <Button id="consent-later" variant="Secondary" fill="None" onClick={later}>
+                  <Text size="B400">Later</Text>
+                </Button>
+              </Box>
+            </Box>
+          </Dialog>
+        </FocusTrap>
+      </OverlayCenter>
+    </Overlay>
+  );
+}