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/ProfileEditor.tsx

@@ -147,7 +147,7 @@ export default function ProfileEditor({ onClose }: { onClose: () => void }) {
       {!formOpen ? (
         <button
           onClick={startNew}
-          className="inline-flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm font-medium bg-accent text-accent-fg hover:opacity-90 transition"
+          className="inline-flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm font-medium btn-primary transition"
         >
           <Plus className="w-4 h-4" /> {t("downloads.profiles.new")}
         </button>
@@ -272,7 +272,7 @@ export default function ProfileEditor({ onClose }: { onClose: () => void }) {
             <button
               onClick={() => save.mutate()}
               disabled={!name.trim() || save.isPending}
-              className="px-3 py-1.5 rounded-lg text-sm font-medium bg-accent text-accent-fg hover:opacity-90 disabled:opacity-50 transition"
+              className="px-3 py-1.5 rounded-lg text-sm font-medium btn-primary disabled:opacity-50 transition"
             >
               {editing ? t("downloads.profiles.save") : t("downloads.profiles.create")}
             </button>