fix(a11y): open a message image from the keyboard (#164)

Timeline images were focusable (tabIndex 0) but only opened on click, so a
keyboard user could Tab to one and do nothing with it, and screen readers
heard a plain image. They are now role="button" labelled "View image: <name>"
and open the viewer on Enter or Space. Focus already returns to the image when
the viewer closes (checked for mouse and keyboard).

Verified in Chromium: Tab → Enter opens the viewer, Escape returns focus to
"View image: red.png", Space opens it again; chromium e2e suite 19 passed.

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

src/app/components/message/content/ImageContent.tsx

@@ -47,6 +47,9 @@ type RenderImageProps = {
   onError: () => void;
   onClick: () => void;
   tabIndex: number;
+  role: 'button';
+  'aria-label': string;
+  onKeyDown: (evt: React.KeyboardEvent<HTMLElement>) => void;
 };
 export type ImageContentProps = {
   body: string;
@@ -188,6 +191,15 @@ export const ImageContent = as<'div', ImageContentProps>(
               onError: handleError,
               onClick: () => (onOpenViewer ? onOpenViewer() : setViewer(true)),
               tabIndex: 0,
+              // [Gitea #164] Keyboard users could Tab to the image but not open it.
+              role: 'button',
+              'aria-label': `View image: ${body}`,
+              onKeyDown: (evt) => {
+                if (evt.key !== 'Enter' && evt.key !== ' ') return;
+                evt.preventDefault();
+                if (onOpenViewer) onOpenViewer();
+                else setViewer(true);
+              },
             })}
           </Box>
         )}