refactor(input): R10 S3b — one input recipe, ring focus everywhere

Unify text inputs onto the form.tsx inputCls look (surface bg, rounded-lg, 2px accent
RING focus), replacing the settings-family border-focus style app-wide:
- deleted 3 duplicate local `const inputCls` (KeyGate/SettingsPanel/SetupWizard) → import
  the shared one; a 4th (Welcome, py-2.5) kept local but re-styled to match.
- ~30 inline settings inputs: default full-size fields → inputCls; size variants (pager/
  table-filter/chat textarea/plex/glass-card) keep their width/radius/padding but swap
  bg-card→bg-surface and border-focus→ring-focus. glass-card/bg-transparent bgs left as-is.
- refreshed the form.tsx doc-comment (the old two-style split is gone).

The ring is the more visible / a11y-correct focus cue, matching R9's focus foundation.

frontend/src/components/Playlists.tsx

@@ -590,7 +590,7 @@ export default function Playlists() {
                     if (e.key === "Escape") setRenaming(false);
                   }}
                   aria-label={t("playlists.rename")}
-                  className="text-lg font-semibold bg-card border border-border rounded-md px-2 py-0.5 outline-hidden focus:border-accent"
+                  className="text-lg font-semibold bg-surface border border-border rounded-md px-2 py-0.5 focus:outline-hidden focus:ring-2 focus:ring-accent/40"
                 />
               ) : (
                 <div className="flex items-center gap-2 min-w-0">