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/lib/RepoTabs.svelte

@@ -58,7 +58,7 @@
           aria-label={language === "de" ? `${repo.name} schließen` : `Close ${repo.name}`}
           title={language === "de" ? "Repository-Tab schließen" : "Close repository tab"}
         >
-          <X size={13} aria-hidden="true" />
+          <X size={11} aria-hidden="true" />
         </button>
       </div>
     {/each}