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/circuit-tree-editor.tsx

@@ -5,6 +5,7 @@ import {
   ArrowDown,
   ArrowDown01,
   ArrowUp,
+  Calculator,
   Check,
   Columns3,
   FilterX,
@@ -160,6 +161,7 @@ import type {
 import { DistributionBoardComponentModal } from "./distribution-board-component-modal";
 import { CircuitGroupModal } from "./circuit-group-modal";
 import { CircuitProtectionModal } from "./circuit-protection-modal";
+import { CableSizingModal } from "./cable-sizing-modal";
 import {
   circuitGroupCategoryLabels,
   type CircuitGroupCategory,
@@ -319,6 +321,8 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
     useState<CircuitGroupEditorIntent | null>(null);
   const [protectionEditorCircuit, setProtectionEditorCircuit] =
     useState<CircuitTreeCircuitDto | null>(null);
+  const [cableSizingEditorCircuit, setCableSizingEditorCircuit] =
+    useState<CircuitTreeCircuitDto | null>(null);
   const [projectDevices, setProjectDevices] = useState<ProjectDeviceDto[]>([]);
   const [isProjectDeviceDrawerOpen, setIsProjectDeviceDrawerOpen] =
     useState(false);
@@ -1439,6 +1443,31 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
     });
   }
 
+  async function handleApplyCableSizing(patch: {
+    cableCrossSection: string;
+    cableType?: string;
+    cableLength?: number;
+  }) {
+    const circuit = cableSizingEditorCircuit;
+    if (!circuit) {
+      return;
+    }
+    await runCommand({
+      label: "Leitungsberechnung übernehmen",
+      redo: async () => {
+        const result = await updateCircuitById(
+          projectId,
+          getExpectedProjectRevision(),
+          circuit.id,
+          patch
+        );
+        applyProjectCommandResult(result);
+        setCableSizingEditorCircuit(null);
+        return null;
+      },
+    });
+  }
+
   async function handleRedo() {
     if (historyBusy || isSaving || !historyState || historyState.redoDepth === 0) {
       return;
@@ -3185,6 +3214,21 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
           onSave={handleSaveCircuitProtection}
         />
       ) : null}
+      {cableSizingEditorCircuit ? (
+        <CableSizingModal
+          circuit={cableSizingEditorCircuit}
+          circuitCategory={
+            data.sections.find(
+              (section) => section.id === cableSizingEditorCircuit.sectionId
+            )?.category
+          }
+          isSaving={isSaving}
+          onApply={handleApplyCableSizing}
+          onClose={() => setCableSizingEditorCircuit(null)}
+          singlePhaseVoltageV={data.singlePhaseVoltageV}
+          threePhaseVoltageV={data.threePhaseVoltageV}
+        />
+      ) : null}
       <div className="editor-toolbar">
         <IconButton
           disabled={!historyState || historyState.undoDepth === 0 || historyBusy || isSaving}
@@ -4530,6 +4574,14 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
                   >
                     {row.circuit && row.rowType !== "deviceRow" ? (
                       <>
+                        <IconButton
+                          disabled={isSaving}
+                          icon={Calculator}
+                          label="Leitungsberechnung"
+                          onClick={() => setCableSizingEditorCircuit(row.circuit!)}
+                          tabIndex={-1}
+                          title="Leitungsberechnung: Querschnitt ermitteln oder Kabel eintragen"
+                        />
                         <IconButton
                           disabled={isSaving}
                           icon={ListPlus}