refactor(ui): address /code-review on .btn-primary — tone family + doc fixes

- Generalize the recipe into a .btn-{tone} family: add .btn-danger / .btn-warn
  siblings sharing one hover-dim, and migrate the two hand-rolled tone-action
  buttons (ConfirmProvider danger confirm, Banner warning action) onto them, so
  primary/danger/warn action paints can't drift.
- Reword the recipe comment: it owns paint + one standard hover-dim (not
  'PAINT-ONLY' — that mislabel obscured the hover behaviour).
- Drop the now-redundant hover:opacity-90 from Banner's action-button base (each
  tone's recipe supplies the hover).

The 3 previously hover-less primary buttons keep the standard hover-dim gained in
the prior commit (intentional consistency; now accurately documented). Gate green
(tsc/eslint-0/prettier/knip/vitest 79); self-E2E confirmed btn-danger/btn-warn
compute to their tone/tone-fg tokens.

frontend/src/components/Banner.tsx

@@ -13,7 +13,7 @@ const TONES: Record<BannerTone, { bar: string; icon: string; action: string }> =
   warning: {
     bar: "bg-warn/15 border-warn/30",
     icon: "text-warn",
-    action: "bg-warn text-warn-fg hover:opacity-90",
+    action: "btn-warn",
   },
 };
 
@@ -42,7 +42,7 @@ export default function Banner({
       {action && (
         <button
           onClick={action.onClick}
-          className={`ml-1 px-2.5 py-1 rounded-lg font-semibold hover:opacity-90 transition ${c.action}`}
+          className={`ml-1 px-2.5 py-1 rounded-lg font-semibold transition ${c.action}`}
         >
           {action.label}
         </button>

frontend/src/components/ConfirmProvider.tsx

@@ -116,7 +116,7 @@ export function ConfirmProvider({ children }: { children: ReactNode }) {
                 onClick={() => close(c.id)}
                 className={`px-4 py-2 rounded-lg text-sm font-semibold transition ${
                   multi ? "w-full" : ""
-                } ${c.danger ? "bg-danger text-danger-fg hover:opacity-90" : "btn-primary"}`}
+                } ${c.danger ? "btn-danger" : "btn-primary"}`}
               >
                 {c.label || t("common.confirm")}
               </button>