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/ChatThread.tsx

@@ -216,7 +216,7 @@ export default function ChatThread({
           <button
             onClick={submit}
             disabled={!draft.trim() || send.isPending}
-            className="shrink-0 p-2.5 rounded-xl bg-accent text-accent-fg hover:opacity-90 transition disabled:opacity-40"
+            className="shrink-0 p-2.5 rounded-xl btn-primary transition disabled:opacity-40"
             aria-label={t("messages.send")}
             title={t("messages.send")}
           >