fix(a11y): mark avatar image as decorative with explicit empty alt

shui/avatar-image was called without an :alt prop, which leaves the
underlying <img> with no alt attribute — some screen readers fall
back to announcing the URL or filename.

The avatar is always rendered next to its label (page/block title),
which the screen reader already announces. Passing the title as alt
would double-announce; the right call is explicit empty alt to mark
the image as decorative per WCAG.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/main/frontend/components/icon.cljs

@@ -579,6 +579,11 @@
      ;; Image (shows when loaded, circular with cover fit)
      (when url
        (shui/avatar-image {:src url
+                           ;; Decorative: the avatar is always rendered next
+                           ;; to its label (page/block title), which the
+                           ;; screen reader already announces. Content-
+                           ;; bearing alt would double-announce.
+                           :alt ""
                            :style {:object-fit "cover"}
                            :data-shape (name shape)}))
      ;; Fallback (shows while loading, on error, OR when there's no image