Add optional cable-sizing module

Per-circuit cross-section recommendation as a separate module attached to
the circuit list. It never writes project data itself: "Übernehmen" applies
cable type, cross-section and length through the regular circuit.update
command, so revisions and Undo/Redo stay intact.

- Pure domain module (src/cable-sizing) with IEC 60364-5-52:2009 Annex B
  tables for PVC and XLPE, copper and aluminium, methods A1, A2, B1, B2, C,
  D1, D2 and E. Every value was cross-checked against public reproductions;
  where sources differ the lower value is used (docs/cable-sizing-module.md).
- Temperature and grouping between table values round to the safer value;
  values outside the tables return no result instead of a factor of 1.
  Ground methods use B.52.18 (D2) and B.52.19 A (D1).
- Overload protection: circuit-breakers In <= Iz; gG fuses additionally
  I2 <= 1.45 Iz with I2 = 1.6/1.9/2.1 In.
- Inputs prefilled from the circuit: power, phase (group or device rows),
  project voltage, power-weighted cos phi, length and protective device.
- Opened from a "Leitungsberechnung" button in the row actions, not from a
  grid cell, so cable cells keep the spreadsheet behavior.
- Stateless API under /api/cable-sizing; no database table or migration.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/server/controllers/cable-sizing.controller.ts

@@ -0,0 +1,37 @@
+import type { Request, Response } from "express";
+import { cableSizingRequestSchema } from "../../cable-sizing/domain/cable-sizing-contracts.js";
+import {
+  buildCableSizingAlerts,
+  calculateCableSizing,
+  INSULATION_MATERIALS,
+  INSULATION_MATERIAL_LABELS,
+  LAYING_METHODS,
+  LAYING_METHOD_LABELS,
+} from "../../cable-sizing/domain/cable-sizing-calculation.js";
+
+// Stateless calculation: it neither reads nor writes project data. The
+// planner applies a result through the existing circuit.update command, which
+// keeps revisions and Undo/Redo intact. See docs/cable-sizing-module.md.
+export function calculateCableSizingHandler(req: Request, res: Response) {
+  const parsed = cableSizingRequestSchema.safeParse(req.body);
+  if (!parsed.success) {
+    return res.status(400).json({ error: parsed.error.flatten() });
+  }
+  const result = calculateCableSizing(parsed.data);
+  return res.json({ result, alerts: buildCableSizingAlerts(parsed.data, result) });
+}
+
+export function listLayingMethodsHandler(_req: Request, res: Response) {
+  return res.json(
+    LAYING_METHODS.map((method) => ({ method, label: LAYING_METHOD_LABELS[method] }))
+  );
+}
+
+export function listInsulationMaterialsHandler(_req: Request, res: Response) {
+  return res.json(
+    INSULATION_MATERIALS.map((insulation) => ({
+      insulation,
+      label: INSULATION_MATERIAL_LABELS[insulation],
+    }))
+  );
+}