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/PlexBrowse.tsx

@@ -673,7 +673,7 @@ function ActionBtn({
       onClick={onClick}
       disabled={disabled}
       className={`inline-flex items-center gap-2 rounded-xl px-3 py-2 text-sm font-medium transition disabled:opacity-50 ${
-        primary ? "bg-accent text-accent-fg shadow-xs hover:opacity-90" : "glass-card glass-hover"
+        primary ? "btn-primary shadow-xs" : "glass-card glass-hover"
       }`}
     >
       <Icon className="w-4 h-4 shrink-0" />