refactor(ui): R10 — .btn-primary recipe for the primary/action buttons

Add a paint-only .btn-primary recipe (solid accent bg + accent-fg text +
hover:opacity dim) in index.css @layer components, alongside the selection-paint
recipes (.pill/.chip/.is-current). Migrate ~38 primary/action buttons across 24
files off the hand-rolled 'bg-accent text-accent-fg hover:opacity-90', keeping
each site's geometry (padding/radius/font/gap/disabled/shadow).

Deliberately NOT migrated: accent BADGES/labels (spans), the chat message bubble,
selection toggles (tableHeader — R8 popover territory), and the round play FAB
(hover:scale, not the opacity-dim primary idiom). Three previously hover-less
primary buttons (SettingsPanel/SetupWizard/DraftSaveBar) gain the standard hover
dim for consistency.

Gate green (tsc/eslint-0/prettier/knip/vitest 79); self-E2E confirmed a migrated
button computes to accent/accent-fg and both recipe rules load.

frontend/src/components/About.tsx

@@ -72,7 +72,7 @@ export default function About({
 
       <button
         onClick={onOpenReleaseNotes}
-        className="mt-4 w-full inline-flex items-center justify-center gap-2 px-4 py-2 rounded-xl font-semibold bg-accent text-accent-fg hover:opacity-90 transition"
+        className="mt-4 w-full inline-flex items-center justify-center gap-2 px-4 py-2 rounded-xl font-semibold btn-primary transition"
       >
         <Sparkles className="w-4 h-4" /> {t("about.whatsNew")}
       </button>