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/ShareDialog.tsx

@@ -216,7 +216,7 @@ function LinkRow({ link, onChanged }: { link: ShareLink; onChanged: () => void }
           <button
             onClick={() => savePass.mutate(newPass.trim())}
             disabled={savePass.isPending || !newPass.trim()}
-            className="px-2 py-1 rounded-md text-xs font-semibold bg-accent text-accent-fg hover:opacity-90 disabled:opacity-40 transition"
+            className="px-2 py-1 rounded-md text-xs font-semibold btn-primary disabled:opacity-40 transition"
           >
             {t("downloads.share.savePassword")}
           </button>
@@ -352,7 +352,7 @@ function LinkShare({ job }: { job: DownloadJob }) {
             <button
               onClick={() => create.mutate()}
               disabled={create.isPending}
-              className={clsx(btnCls, "bg-accent text-accent-fg hover:opacity-90")}
+              className={clsx(btnCls, "btn-primary")}
             >
               {t("downloads.share.createLink")}
             </button>