Add optional room areas

src/frontend/utils/room-area.ts

@@ -0,0 +1,30 @@
+export type RoomAreaParseResult =
+  | { ok: true; value: number | null }
+  | { ok: false; error: string };
+
+/** Parses an optional German decimal without rounding the stored value. */
+export function parseRoomAreaInput(text: string): RoomAreaParseResult {
+  const normalized = text.trim();
+  if (!normalized) return { ok: true, value: null };
+  if (!/^(?:\d+(?:,\d*)?|,\d+)$/.test(normalized)) {
+    return {
+      ok: false,
+      error: "Bitte eine nicht negative Zahl mit Komma eingeben.",
+    };
+  }
+  const value = Number(normalized.replace(",", "."));
+  return Number.isFinite(value) && value >= 0
+    ? { ok: true, value }
+    : { ok: false, error: "Die Fläche muss mindestens 0 m² betragen." };
+}
+
+export function formatRoomArea(areaM2: number): string {
+  return areaM2.toLocaleString("de-DE", {
+    minimumFractionDigits: 2,
+    maximumFractionDigits: 2,
+  });
+}
+
+export function formatOptionalRoomAreaInput(areaM2: number | null | undefined): string {
+  return areaM2 === null || areaM2 === undefined ? "" : formatRoomArea(areaM2);
+}