fix(design): the button and chip hover colours were never generated — an act: variant instead of a runtime prefix (E28)

buttonStyles/chipStyles glued the hover guard on at runtime (`${ACT}hover:text-fg`). Tailwind emits CSS
only for a class it finds whole in the source, so all 22 guarded hovers shipped with no rule: no text or
edge colour on hover for ghost/outline/danger buttons, no white layer on the player's control row, no
underline on a link button, no accent hover on token/suggest/ghost chips. The stylesheet had 0 rules
for `not-aria-disabled`.

- `@custom-variant act` (index.css): the same guard as the state layers; `:disabled` never matches an
  <a>, so one variant serves buttons and links and the per-element variant maps go.
- ui/tailwindScan.contract.test.ts: every class the primitives can emit stands whole in their source.
- a lint rule refuses a variant right after an interpolation; the `enabled:hover:` rule now points at
  `act:hover:`.

frontend/eslint.config.js

@@ -372,6 +372,11 @@ const PALETTE =
 const PALETTE_MESSAGE =
   "a palette colour: use a semantic token (text-rating, bg-live, bg-media-chrome, text-danger, …; DESIGN.md Tokens), not text-amber-500.";
 
+// A class name split by an interpolation: the text right after `${…}` starts with a variant (`hover:`).
+const SPLIT_CLASS = "/^[a-z][\\w-]*:[\\w\\[!-]/";
+const SPLIT_CLASS_MESSAGE =
+  "a class split by an interpolation (`${x}hover:…`): Tailwind never sees it whole and emits no CSS; write the class literally (a shared guard is the `act:` variant).";
+
 export default tseslint.config(
   { ignores: ["dist", "node_modules", "playwright-report", "test-results", "e2e/.auth"] },
   {
@@ -451,12 +456,19 @@ export default tseslint.config(
         {
           selector: "Literal[value=/(^|\\s)enabled:hover:/]",
           message:
-            "`enabled:hover:` misses a focused FocusSafeButton (aria-disabled): use `enabled:not-aria-disabled:hover:`.",
+            "`enabled:hover:` misses a focused FocusSafeButton (aria-disabled): use `act:hover:` (index.css).",
         },
         {
           selector: "TemplateElement[value.raw=/(^|\\s)enabled:hover:/]",
           message:
-            "`enabled:hover:` misses a focused FocusSafeButton (aria-disabled): use `enabled:not-aria-disabled:hover:`.",
+            "`enabled:hover:` misses a focused FocusSafeButton (aria-disabled): use `act:hover:` (index.css).",
+        },
+        // Tailwind emits CSS only for a class it finds WHOLE in the source. A variant glued onto an
+        // interpolated prefix (`${ACT}hover:text-fg`) is never found, so it ships with no rule: every
+        // hover colour of the button and chip primitives was dead that way (E28 audit).
+        {
+          selector: `TemplateLiteral > TemplateElement:not(:first-child)[value.raw=${SPLIT_CLASS}]`,
+          message: SPLIT_CLASS_MESSAGE,
         },
         // The canonical "FE control" row (CLAUDE.md): a short fixed set is a SegmentedToggle, a
         // longer one or a user's own list a FilterSelect. Six native selects had drifted in; the