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/ui/DraftSaveBar.tsx

@@ -71,7 +71,7 @@ export function DraftSaveBar({
           data-testid="draft-save"
           onClick={onSave}
           disabled={saving || !dirty}
-          className="flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm bg-accent text-accent-fg font-medium shadow-md shadow-accent/20 disabled:opacity-40 transition"
+          className="flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm btn-primary font-medium shadow-md shadow-accent/20 disabled:opacity-40 transition"
         >
           <Save className="w-4 h-4" />
           {saving ? labels.saving : labels.save}