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/AdminUsers.tsx

@@ -347,7 +347,7 @@ function AdminInvites() {
           value={newEmail}
           onChange={(e) => setNewEmail(e.target.value)}
           placeholder={t("settings.invites.addPlaceholder")}
-          className="flex-1 min-w-0 bg-card border border-border rounded-xl px-3 py-2 text-sm outline-hidden focus:border-accent"
+          className="flex-1 min-w-0 bg-surface border border-border rounded-xl px-3 py-2 text-sm focus:outline-hidden focus:ring-2 focus:ring-accent/40"
         />
         <button
           type="submit"
@@ -466,7 +466,7 @@ function AdminDemo() {
           value={newEmail}
           onChange={(e) => setNewEmail(e.target.value)}
           placeholder={t("settings.demo.addPlaceholder")}
-          className="flex-1 min-w-0 bg-card border border-border rounded-xl px-3 py-2 text-sm outline-hidden focus:border-accent"
+          className="flex-1 min-w-0 bg-surface border border-border rounded-xl px-3 py-2 text-sm focus:outline-hidden focus:ring-2 focus:ring-accent/40"
         />
         <button
           type="submit"