fix(web): unify display-field spacing (shared Field) + clearer layout help

- Every option in the custom form now renders through one `Field` wrapper (label →
  control → help at a consistent `space-y-3`), so the label→input gap is roomier and
  identical across keep-alive, the button groups, and max-displays — the first field no
  longer spaces differently from the rest.
- Reworded the multi-monitor layout help: it now says Auto is side-by-side and Manual
  gives a per-display X/Y editor "in the Live displays section below once two or more are
  streaming" — instead of pointing at an "arrangement table" that isn't visible until
  clients connect.

web tsc + vite build + biome-lint green; deployed on .21.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/messages/de.json

@@ -100,7 +100,7 @@
 	"display_identity_per_client": "Pro Client",
 	"display_identity_per_client_mode": "Pro Client + Auflösung",
 	"display_layout_mode": "Multi-Monitor-Anordnung",
-	"display_layout_help": "Wie mehrere Anzeigen auf dem Desktop angeordnet werden. Manuell nutzt die Anordnungstabelle unten (ab 2 Anzeigen).",
+	"display_layout_help": "Automatisch ordnet die Anzeigen nebeneinander an (links nach rechts). Manuell: Du platzierst jede selbst — ein X/Y-Editor pro Anzeige erscheint im Abschnitt „Aktive Displays“ unten, sobald zwei oder mehr streamen.",
 	"display_layout_auto_row": "Automatisch (nebeneinander)",
 	"display_layout_manual": "Manuell",
 	"clients_title": "Gekoppelte Geräte",