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/features/room/RoomTimeline.tsx

@@ -1235,6 +1235,7 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
                 outlineAttachment={messageLayout === MessageLayout.Bubble}
                 eventId={mEventId}
                 onOpenImageViewer={() => setLightboxEventId(mEventId)}
+                mEvent={mEvent}
               />
             )}
           </Message>
@@ -1365,6 +1366,7 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
                       outlineAttachment={messageLayout === MessageLayout.Bubble}
                       eventId={mEventId}
                       onOpenImageViewer={() => setLightboxEventId(mEventId)}
+                      mEvent={mEvent}
                     />
                   );
                 }

src/app/features/room/thread/ThreadTimeline.tsx

@@ -715,6 +715,7 @@ export function ThreadTimeline({ room, thread, editor }: ThreadTimelineProps) {
             outlineAttachment={messageLayout === MessageLayout.Bubble}
             eventId={mEventId}
             onOpenImageViewer={() => setLightboxEventId(mEventId)}
+            mEvent={mEvent}
           />
         );
       };