feat(button): add link variant and polymorphic <a> rendering

When :href is provided, the button renders as <a> instead of <button>,
enabling navigation links with full button styling.

New :link variant renders as a minimal text link (underlined, accent
color, no background/padding) — works as both <button> and <a>.

CSS includes reset styles for a.btn (removes default link decoration)
and preserves underline for a.btn-link.

bb.edn

@@ -30,6 +30,7 @@
               [ui.tooltip-test]
               [ui.breadcrumb-test]
               [ui.pagination-test]
+              [ui.form-test]
               [ui.theme-test])
    :task (let [{:keys [fail error]} (t/run-tests
                                       'ui.button-test
@@ -46,6 +47,7 @@
                                       'ui.tooltip-test
                                       'ui.breadcrumb-test
                                       'ui.pagination-test
+                                      'ui.form-test
                                       'ui.theme-test)]
            (when (pos? (+ fail error))
              (System/exit 1)))}