fix(tailwind): support boolean data-selected attribute from Base UI

[codewithsupra]

Problem

@custom-variant data-selected in packages/shadcn/src/tailwind.css only matched [data-selected="true"], but Base UI sets data-selected as a boolean attribute (bare presence, no value) — not as ="true".

This means data-selected:font-medium and similar utilities had no effect on Base UI's Combobox, Select, and other components that use the attribute selector.

All other @custom-variant definitions in this file already handle both forms:

Variant	Boolean form
data-checked	✅ [data-checked]:not([data-checked="false"])
data-disabled	✅ [data-disabled]:not([data-disabled="false"])
data-active	✅ [data-active]:not([data-active="false"])
data-selected	❌ only [data-selected="true"] ← this PR

Fix

 @custom-variant data-selected {
-  &:where([data-selected="true"]) {
+  &:where([data-selected="true"]),
+  &:where([data-selected]:not([data-selected="false"])) {
     @slot;
   }
 }

This matches the exact same pattern used by data-checked, data-disabled, and data-active.

Fixes #11038

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