feat(e2ee): undecryptable placeholder says why and offers the fix (#159)

'Unable to decrypt message' now carries one sentence per matrix-js-sdk
DecryptionFailureCode (describeDecryptionFailure, unit-tested against every
code so no raw code can leak into the copy) and, where something fixes it,
one button: no key backup → 'Set up key backup'; backup exists but this
session can't open it / key withheld for an unverified session → 'Unlock key
backup' / 'Verify this session' (both open Settings → Devices via a new
settingsRequestAtom that SettingsTab consumes); backup working or unknown
session (rust-crypto re-requests keys itself) → 'Retry', which re-runs
decryptEventIfNeeded. Sender-side problems are plain text. The raw code sits
in the placeholder's tooltip for support.

Verified headless on a fresh session in the encrypted seed room: each event
shows 'Sent before you signed in here, and no key backup exists…' with
tooltip HISTORICAL_MESSAGE_NO_KEY_BACKUP; the button opens Settings → Devices.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/RenderMessageContent.tsx

@@ -1,5 +1,5 @@
 import React from 'react';
-import { MsgType } from 'matrix-js-sdk';
+import { MatrixEvent, MsgType } from 'matrix-js-sdk';
 import { HTMLReactParserOptions } from 'html-react-parser';
 import { Opts } from 'linkifyjs';
 import { config, Text } from 'folds';
@@ -72,6 +72,8 @@ type RenderMessageContentProps = {
   eventId?: string;
   /** [Gitea #219] Open the room's shared media lightbox at this event. */
   onOpenImageViewer?: () => void;
+  /** [Gitea #159] The event, for the undecryptable placeholder's reason + retry. */
+  mEvent?: MatrixEvent;
 };
 export function RenderMessageContent({
   displayName,
@@ -88,6 +90,7 @@ export function RenderMessageContent({
   outlineAttachment,
   eventId,
   onOpenImageViewer,
+  mEvent,
 }: RenderMessageContentProps) {
   const renderUrlsPreview = (urls: string[]) => {
     // Cap previews per message so a link-dump doesn't spawn dozens of preview
@@ -346,7 +349,7 @@ export function RenderMessageContent({
   }
 
   if (msgType === 'm.bad.encrypted') {
-    return <MBadEncrypted />;
+    return <MBadEncrypted mEvent={mEvent} />;
   }
 
   if (msgType === 'm.key.verification.request') {