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/VideoEditor.tsx

@@ -630,10 +630,7 @@ export default function VideoEditor({ job, onClose }: { job: DownloadJob; onClos
             <button
               onClick={() => create.mutate()}
               disabled={!valid || create.isPending}
-              className={clsx(
-                btnCls,
-                "bg-accent text-accent-fg hover:opacity-90 inline-flex items-center gap-1.5",
-              )}
+              className={clsx(btnCls, "btn-primary inline-flex items-center gap-1.5")}
             >
               {create.isPending ? (
                 <Loader2 className="w-4 h-4 animate-spin" />