style(repo-tabs): refine close button layout and focus state

Improve sizing and interaction of the repository tab close button. The
control now has a consistent 24px circular hit area. Hover and
focus-visible states use a subtle accent color for clarity. Icon size
is reduced to better center the glyph inside the control.

- Make the close button a 24px circular, grid-centered element.
- Add focus-visible and hover color with a subtle accent background.
- Reduce the close icon to 11px to improve visual balance.

src/app.css

@@ -1112,10 +1112,28 @@
     background: transparent;
     color: var(--color-ink-faint);
   }
-  .repo-tab-close { opacity: 0.58; transition: opacity 120ms ease, color 120ms ease, background 120ms ease; }
+  .repo-tab-close {
+    display: grid;
+    place-items: center;
+    align-self: center;
+    min-width: 24px;
+    width: 24px;
+    min-height: 24px;
+    height: 24px;
+    flex: 0 0 24px;
+    aspect-ratio: 1;
+    margin-right: 6px;
+    border-radius: 50%;
+    opacity: 0.58;
+    transition: opacity 120ms ease, color 120ms ease, background 120ms ease;
+  }
   .repo-tab-wrap:hover .repo-tab-close,
   .repo-tab-wrap.active .repo-tab-close { opacity: 1; }
-  .repo-tab-close:hover:not(:disabled) { color: var(--color-ink); background: var(--color-surface-hover); }
+  .repo-tab-close:hover:not(:disabled),
+  .repo-tab-close:focus-visible:not(:disabled) {
+    color: #ff6673;
+    background: rgba(255, 90, 103, 0.13);
+  }
   .repo-tab-add {
     min-width: 44px;
     border-left: 1px solid var(--color-border-subtle);