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/index.ts

@@ -2,6 +2,7 @@ import express from "express";
 import { globalDeviceRouter } from "./routes/global-device.routes.js";
 import { projectDeviceRouter } from "./routes/project-device.routes.js";
 import { projectRouter } from "./routes/project.routes.js";
+import { cableSizingRouter } from "./routes/cable-sizing.routes.js";
 import { errorMiddleware } from "./middleware/error.middleware.js";
 import { createLogger, toErrorMeta } from "../shared/logging/logger.js";
 import { closeDefaultDatabase } from "../db/client.js";
@@ -48,6 +49,7 @@ app.get("/health", (_req, res) => {
   res.json({ ok: true });
 });
 
+app.use("/api/cable-sizing", cableSizingRouter);
 app.use("/api/projects", projectRouter);
 app.use("/api/global-devices", globalDeviceRouter);
 app.use("/api/project-devices", projectDeviceRouter);