Add proposed cable-sizing module

Isolated module (mirrors src/external-model/ dependency direction) that
adds an on-demand, DIN VDE 0298-4 referenced cable cross-section
calculator: a click-on-a-circuit input mask for laying method,
insulation, ambient temperature, grouping and voltage-drop limit that
suggests a cross-section for the circuit cableCrossSection/cableLength
fields. Applying a suggestion goes through the existing circuit.update
command (expectedRevision, undo/redo) unchanged - nothing here writes
to circuits directly or touches the revision/command system. See
docs/cable-sizing-module.md for the full rationale, API contract,
verification status and maintenance plan. Proposal, not yet reviewed
by the project owner - see the docs file.

423 existing + 11 new tests pass, build:api/build:web/typecheck:scripts
clean.

src/frontend/components/cable-sizing-api.ts

@@ -0,0 +1,71 @@
+// Thin, self-contained fetch client for the cable-sizing module. Deliberately
+// not merged into ../utils/api.ts to keep this module's frontend footprint a
+// single new file rather than growing the existing shared API file - see
+// docs/cable-sizing-module.md.
+import type {
+  CableSizingAlert,
+  CableSizingInput,
+  CableSizingResult,
+  InsulationMaterial,
+  LayingMethod,
+} from "../../cable-sizing/domain/cable-sizing-calculation";
+
+async function request<T>(url: string, init?: RequestInit): Promise<T> {
+  const response = await fetch(url, {
+    ...init,
+    headers: { "Content-Type": "application/json", ...init?.headers },
+    cache: "no-store",
+  });
+  if (!response.ok) {
+    let details = await response.text();
+    try {
+      const parsed = JSON.parse(details) as { error?: unknown };
+      if (typeof parsed.error === "string") details = parsed.error;
+    } catch {
+      // keep raw text
+    }
+    throw new Error(details || `Anfrage fehlgeschlagen (Status ${response.status})`);
+  }
+  return response.json() as Promise<T>;
+}
+
+export interface CalculateCableSizingResponse {
+  calculationId: string;
+  result: CableSizingResult;
+  alerts: CableSizingAlert[];
+}
+
+export function calculateCableSizing(
+  input: CableSizingInput,
+  context?: { projectId?: string; circuitId?: string; equipmentIdentifier?: string }
+): Promise<CalculateCableSizingResponse> {
+  return request("/api/cable-sizing/calculate", {
+    method: "POST",
+    body: JSON.stringify({ ...input, context }),
+  });
+}
+
+export function markCableSizingCalculationApplied(calculationId: string): Promise<unknown> {
+  return request(`/api/cable-sizing/calculations/${calculationId}/applied`, {
+    method: "POST",
+  });
+}
+
+export interface LayingMethodOption {
+  method: LayingMethod;
+  label: string;
+  dataVerified: boolean;
+}
+
+export function listLayingMethods(): Promise<LayingMethodOption[]> {
+  return request("/api/cable-sizing/laying-methods");
+}
+
+export interface InsulationMaterialOption {
+  insulation: InsulationMaterial;
+  label: string;
+}
+
+export function listInsulationMaterials(): Promise<InsulationMaterialOption[]> {
+  return request("/api/cable-sizing/insulation-materials");
+}