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/Welcome.tsx

@@ -501,7 +501,7 @@ function AuthCard() {
             <button
               type="submit"
               disabled={busy}
-              className="px-4 py-2.5 rounded-xl font-semibold bg-accent text-accent-fg hover:opacity-90 disabled:opacity-50 transition"
+              className="px-4 py-2.5 rounded-xl font-semibold btn-primary disabled:opacity-50 transition"
             >
               {busy ? t("welcome.auth.working") : t(SUBMIT_KEY[mode])}
             </button>