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/PlayerModal.tsx

@@ -1088,7 +1088,7 @@ export default function PlayerModal({
                     href={`https://www.youtube.com/watch?v=${currentVideoId}`}
                     target="_blank"
                     rel="noreferrer"
-                    className="inline-flex items-center justify-center gap-2 px-4 py-2 rounded-xl font-semibold bg-accent text-accent-fg hover:opacity-90 transition"
+                    className="inline-flex items-center justify-center gap-2 px-4 py-2 rounded-xl font-semibold btn-primary transition"
                   >
                     <ExternalLink className="w-4 h-4" />
                     {t("player.openOnYoutube")}
@@ -1314,7 +1314,7 @@ export default function PlayerModal({
                   className={
                     "shrink-0 inline-flex items-center gap-1.5 text-sm px-3 py-1.5 rounded-lg transition " +
                     (watched
-                      ? "bg-accent text-accent-fg shadow-xs hover:opacity-90"
+                      ? "btn-primary shadow-xs"
                       : "text-muted hover:text-fg hover:bg-surface border border-border")
                   }
                 >