Add the rough power balance module for early design phases

In LPH 2/3 there are no circuits yet, but the office needs a power
balance by DIN 276 cost group, as in the Excel template "Leistungsbilanz -
KG400": each trade enters consumers by piece or by area (W/m²) with GLF
and cos phi; the balance sums AV and SV per cost group, adds a reserve and
derives the maximum current.

The module is separate from the circuit structure: own tables
(migration 0010), own folder src/estimate-balance and an import-free
shared calculation used by API and UI. Writes go through project commands
(estimate-balance.insert/.update/.delete with complete documents), so
revision checks and Undo/Redo apply; it is not part of project snapshots
or transfer yet. A comparison endpoint sums the device rows of all circuit
lists by cost group (longest prefix) against the estimate.

The UI adds a sidebar entry, a list of Stände (template KG 400, empty or
copy) and a spreadsheet-like editor with autosave, AMEV picker for the
GLF, totals, reserve, maximum current, comparison table and print layout.
The tests reproduce the template example exactly (143.24 kW, 148.48 kVA,
157.56 kW / 163.33 kVA with 10 % reserve, 235.75 A). See
docs/estimate-balance.md and issue #7.

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

src/frontend/estimate-balance/estimate-draft-input.tsx

@@ -0,0 +1,107 @@
+"use client";
+
+import { useEffect, useRef, useState } from "react";
+
+export type DraftParseResult<T> = { ok: true; value: T } | { ok: false; error: string };
+
+interface DraftInputProps<T> {
+  ariaLabel: string;
+  className?: string;
+  disabled?: boolean;
+  format: (value: T) => string;
+  inputMode?: "decimal" | "text";
+  maxLength?: number;
+  onCommit: (value: T) => void;
+  /** Reports whether the raw text is currently valid (false while invalid). */
+  onValidityChange?: (isValid: boolean) => void;
+  parse: (text: string) => DraftParseResult<T>;
+  placeholder?: string;
+  value: T;
+}
+
+/**
+ * Input that keeps the raw text the user typed. Only a valid text is
+ * committed into the document; an invalid text stays visible with a red
+ * border and is never saved. A value changed from outside (reload, undo)
+ * replaces the text unless the text already represents that value.
+ */
+export function DraftInput<T>({
+  ariaLabel,
+  className = "estimate-cell-input",
+  disabled,
+  format,
+  inputMode = "text",
+  maxLength,
+  onCommit,
+  onValidityChange,
+  parse,
+  placeholder,
+  value,
+}: DraftInputProps<T>) {
+  const [text, setText] = useState(() => format(value));
+  const [error, setError] = useState<string | null>(null);
+  const [syncedValue, setSyncedValue] = useState(value);
+
+  if (!Object.is(syncedValue, value)) {
+    // Adjusting state while rendering (instead of an effect) avoids a frame
+    // with stale text after an external change.
+    setSyncedValue(value);
+    const parsed = parse(text);
+    if (!parsed.ok || !Object.is(parsed.value, value)) {
+      setText(format(value));
+      setError(null);
+    }
+  }
+
+  const validityRef = useRef(onValidityChange);
+  useEffect(() => {
+    validityRef.current = onValidityChange;
+  });
+  const isValid = error === null;
+  useEffect(() => {
+    validityRef.current?.(isValid);
+  }, [isValid]);
+  // A removed input (deleted row) must not leave an invalid marker behind.
+  useEffect(() => () => validityRef.current?.(true), []);
+
+  function handleChange(nextText: string) {
+    setText(nextText);
+    const parsed = parse(nextText);
+    if (!parsed.ok) {
+      setError(parsed.error);
+      return;
+    }
+    setError(null);
+    if (!Object.is(parsed.value, value)) {
+      setSyncedValue(parsed.value);
+      onCommit(parsed.value);
+    }
+  }
+
+  function handleBlur() {
+    // Show the canonical notation again ("0,80" → "0,8") once editing ends.
+    if (error === null) {
+      setText(format(value));
+    }
+  }
+
+  return (
+    <>
+      <input
+        aria-invalid={error ? true : undefined}
+        aria-label={ariaLabel}
+        className={error ? `${className} is-invalid` : className}
+        disabled={disabled}
+        inputMode={inputMode}
+        maxLength={maxLength}
+        onBlur={handleBlur}
+        onChange={(event) => handleChange(event.target.value)}
+        placeholder={placeholder}
+        title={error ?? undefined}
+        type="text"
+        value={text}
+      />
+      <span className="estimate-print-value">{error ? format(value) : text}</span>
+    </>
+  );
+}