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/frontend/components/cable-sizing-defaults.ts

@@ -0,0 +1,54 @@
+// Derives the cable-sizing inputs that follow from the circuit itself, so the
+// planner starts from the circuit's real values instead of fixed defaults.
+import type {
+  CircuitCategory,
+  ExistingProtection,
+} from "../../cable-sizing/domain/cable-sizing-calculation";
+import type { CircuitTreeCircuitDto, CircuitTreeProtectionDeviceDto } from "../types";
+
+/** Three-phase if the circuit group or any device row is three-phase. */
+export function deriveCircuitPhase(
+  circuit: CircuitTreeCircuitDto,
+  circuitCategory?: CircuitCategory
+): 1 | 3 {
+  return circuitCategory === "three_phase" ||
+    circuit.deviceRows.some((row) => row.phaseType === "three_phase")
+    ? 3
+    : 1;
+}
+
+/**
+ * Power-weighted cos φ of the circuit: P / S with S = Σ P_row / cos φ_row.
+ * Rows without cos φ count as 1; a circuit without load yields 1.
+ */
+export function deriveCircuitCosPhi(circuit: CircuitTreeCircuitDto): number {
+  let activePowerKw = 0;
+  let apparentPowerKva = 0;
+  for (const row of circuit.deviceRows) {
+    activePowerKw += row.rowTotalPower;
+    apparentPowerKva += row.rowTotalPower / (row.cosPhi || 1);
+  }
+  if (activePowerKw <= 0 || apparentPowerKva <= 0) {
+    return 1;
+  }
+  return Math.round((activePowerKw / apparentPowerKva) * 100) / 100;
+}
+
+/**
+ * Maps the circuit's protective device to the overload-protection kind of the
+ * calculation. Circuit-breakers (LS, FI/LS, AFDD) are "breaker", D0 and NH
+ * fuse links "fuse"; a residual-current device without overcurrent release
+ * (FI) provides no overload protection and is ignored.
+ */
+export function toExistingProtection(
+  protection: CircuitTreeProtectionDeviceDto | undefined
+): ExistingProtection | undefined {
+  if (!protection || protection.type === "FI") {
+    return undefined;
+  }
+  const kind =
+    protection.type === "LS" || protection.type === "FI_LS" || protection.type === "AFDD"
+      ? "breaker"
+      : "fuse";
+  return { ratedCurrentA: protection.ratedCurrentA, kind };
+}