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/ChannelPage.tsx

@@ -460,7 +460,7 @@ export default function ChannelPage({
                   <button
                     onClick={() => subscribe.mutate()}
                     disabled={subscribe.isPending}
-                    className="inline-flex items-center gap-1.5 px-3 py-2 rounded-lg text-sm bg-accent text-accent-fg hover:opacity-90 disabled:opacity-50"
+                    className="inline-flex items-center gap-1.5 px-3 py-2 rounded-lg text-sm btn-primary disabled:opacity-50"
                   >
                     <Plus className="w-4 h-4" />
                     {t("channel.subscribe")}