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/PlexSidebar.tsx

@@ -171,7 +171,7 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
               onChange={(e) => setNewPlaylist(e.target.value)}
               onKeyDown={(e) => e.key === "Enter" && createPlaylist()}
               placeholder={t("plex.playlist.newPlaceholder")}
-              className="glass-card min-w-0 flex-1 rounded-lg px-2 py-1 text-xs outline-hidden focus:border-accent"
+              className="glass-card min-w-0 flex-1 rounded-lg px-2 py-1 text-xs focus:outline-hidden focus:ring-2 focus:ring-accent/40"
             />
             <button
               onClick={createPlaylist}
@@ -298,7 +298,7 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
             value={collSearch}
             onChange={(e) => setCollSearch(e.target.value)}
             placeholder={t("plex.filter.collectionSearch")}
-            className="mb-1.5 w-full rounded-lg border border-border bg-card px-2 py-1 text-sm focus:border-accent focus:outline-hidden"
+            className="mb-1.5 w-full rounded-lg border border-border bg-surface px-2 py-1 text-sm focus:outline-hidden focus:ring-2 focus:ring-accent/40"
           />
           <div
             ref={collFade.ref}
@@ -546,7 +546,7 @@ function NumberInput({
         const n = e.target.value === "" ? null : Number(e.target.value);
         onChange(n != null && Number.isFinite(n) ? n : null);
       }}
-      className="w-20 rounded-lg border border-border bg-card px-2 py-1 text-sm tabular-nums focus:border-accent focus:outline-hidden"
+      className="w-20 rounded-lg border border-border bg-surface px-2 py-1 text-sm tabular-nums focus:outline-hidden focus:ring-2 focus:ring-accent/40"
     />
   );
 }