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/utils/consent.ts

@@ -0,0 +1,21 @@
+import { EventStatus, MatrixEvent } from 'matrix-js-sdk';
+
+export const CONSENT_ERRCODE = 'M_CONSENT_NOT_GIVEN';
+
+/**
+ * The consent page comes from whichever homeserver the user is on (this client
+ * works with any of them), so only ever open a plain web link.
+ */
+export const safeConsentUri = (uri: unknown): string | undefined => {
+  if (typeof uri !== 'string') return undefined;
+  try {
+    const url = new URL(uri);
+    return url.protocol === 'https:' || url.protocol === 'http:' ? url.href : undefined;
+  } catch {
+    return undefined;
+  }
+};
+
+/** True when a local echo failed because the server wants the ToS accepted. */
+export const failedForConsent = (event: MatrixEvent): boolean =>
+  event.status === EventStatus.NOT_SENT && event.error?.errcode === CONSENT_ERRCODE;