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/PlaylistsRail.tsx

@@ -154,7 +154,7 @@ export default function PlaylistsRail({
                 value={plSort.key}
                 onChange={(e) => setPlSort({ ...plSort, key: e.target.value as PlSort["key"] })}
                 aria-label={t("playlists.sortLabel")}
-                className="flex-1 min-w-0 bg-card border border-border rounded-md px-1.5 py-1 text-2xs outline-hidden focus:border-accent"
+                className="flex-1 min-w-0 bg-surface border border-border rounded-md px-1.5 py-1 text-2xs focus:outline-hidden focus:ring-2 focus:ring-accent/40"
               >
                 <option value="custom">{t("playlists.railSortCustom")}</option>
                 <option value="name">{t("playlists.railSortName")}</option>
@@ -200,7 +200,7 @@ export default function PlaylistsRail({
               onChange={(e) => setNewName(e.target.value)}
               placeholder={t("playlists.newPlaylist")}
               disabled={!canWrite}
-              className="flex-1 min-w-0 bg-card border border-border rounded-md px-2 py-1 text-xs outline-hidden focus:border-accent disabled:opacity-50"
+              className="flex-1 min-w-0 bg-surface border border-border rounded-md px-2 py-1 text-xs focus:outline-hidden focus:ring-2 focus:ring-accent/40 disabled:opacity-50"
             />
           </form>
         </div>