feat(media): timeline images open the gallery lightbox at that event (#219)

Clicking an image in the room or thread timeline now opens the same viewer
the media gallery uses — dark backdrop, sender + date, 1/N counter, ←/→ across
the room's media, +/-/0 and wheel/double-click zoom, download, jump to
message — positioned at the clicked event. RoomMediaLightbox feeds it from the
detached media timeline (#163); when the event isn't in the loaded window it
pages back (bounded, 6 pages) and shows the clicked image alone meanwhile, so
the viewer opens instantly. ImageContent gains onOpenViewer (RenderMessageContent
passes onOpenImageViewer); its built-in viewer remains for stickers, search
results, pins, notifications and avatars.

Verified headless: click → 'Media viewer' dialog focused, counter 1/2, '+' →
120 %, Jump to message present, Esc closes; an older image 10 pages up →
found at 10/45 after paging.

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

src/app/components/RenderMessageContent.tsx

@@ -70,6 +70,8 @@ type RenderMessageContentProps = {
   linkifyOpts: Opts;
   outlineAttachment?: boolean;
   eventId?: string;
+  /** [Gitea #219] Open the room's shared media lightbox at this event. */
+  onOpenImageViewer?: () => void;
 };
 export function RenderMessageContent({
   displayName,
@@ -85,6 +87,7 @@ export function RenderMessageContent({
   linkifyOpts,
   outlineAttachment,
   eventId,
+  onOpenImageViewer,
 }: RenderMessageContentProps) {
   const renderUrlsPreview = (urls: string[]) => {
     // Cap previews per message so a link-dump doesn't spawn dozens of preview
@@ -241,6 +244,7 @@ export function RenderMessageContent({
               autoPlay={mediaAutoLoad}
               renderImage={(p) => <Image {...p} loading="lazy" />}
               renderViewer={(p) => <ImageViewer {...p} />}
+              onOpenViewer={onOpenImageViewer}
             />
           )}
           outlined={outlineAttachment}