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/ConfirmProvider.tsx

@@ -116,11 +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"
-                    : "bg-accent text-accent-fg hover:opacity-90"
-                }`}
+                } ${c.danger ? "bg-danger text-danger-fg hover:opacity-90" : "btn-primary"}`}
               >
                 {c.label || t("common.confirm")}
               </button>