Add in-app help, formula reference outlines, underline and auto-closing

- Help dialog (?, F1, Datei > Hilfe): user topics, all shortcuts and the
  formula functions with one search over everything. Topics and shortcuts
  are plain data in data/help.ts; the function list comes from
  codeReference.ts so new built-ins appear by themselves.
- While a formula is edited, the grid outlines each referenced cell/range
  in the colour of its position in the text, and the outline can be
  dragged to move or resize the reference, as in Excel. Point-mode picks
  (click, drag, arrow keys) are outlined too, so keyboard picking is no
  longer blind.
- Underline as a cell format, plus Mod+Shift+B/I/U toggles that switch on
  unless every selected cell already has the format.
- A formula committed with open parentheses or an open text literal gets
  the missing closers appended; anything that still doesn't parse is left
  alone so the usual error shows instead of a guess.
- Cells and tables with a fill but no text colour get dark or light text
  by WCAG contrast instead of the theme colour (no black on black).

app/frontend/engine/__tests__/refs.spec.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-import { parseA1, toA1, cycleReferenceLocks, currentIdentifierAtCaret } from '../refs';
+import { parseA1, toA1, cycleReferenceLocks, currentIdentifierAtCaret, formulaRefSpans } from '../refs';
 
 describe('parseA1 / toA1: $-locked (absolute/mixed) references', () => {
   it('parseA1 reports which axes are $-locked', () => {
@@ -121,3 +121,30 @@ describe('currentIdentifierAtCaret -- function-name autocomplete detection', ()
     expect(currentIdentifierAtCaret('=SUM("x', 7)).toBeNull();
   });
 });
+
+describe('formulaRefSpans -- references with their position in the text', () => {
+  const texts = (f: string) => formulaRefSpans(f).map((s) => f.slice(s.start, s.end));
+
+  it('finds cells, ranges and whole lines in text order', () => {
+    expect(texts('=SUMME(A1:B3;$C$2)*D4+B:B')).toEqual(['A1:B3', '$C$2', 'D4', 'B:B']);
+    const [r] = formulaRefSpans('=A1:$B$3');
+    expect(r.a).toEqual({ col: 0, row: 0, colAbs: false, rowAbs: false });
+    expect(r.b).toEqual({ col: 1, row: 2, colAbs: true, rowAbs: true });
+    expect(formulaRefSpans('=2:5')[0]).toMatchObject({ whole: 'row', a: { row: 1 }, b: { row: 4 } });
+  });
+
+  it('includes the qualifier in the span and reports it separately', () => {
+    const f = "='Kosten Q3'!A1+Analyse!Umsatz::B2:C4";
+    expect(texts(f)).toEqual(["'Kosten Q3'!A1", 'Analyse!Umsatz::B2:C4']);
+    const [q, s] = formulaRefSpans(f);
+    expect(q).toMatchObject({ qualifier: 'Kosten Q3', refStart: f.indexOf('A1') });
+    expect(s).toMatchObject({ qualifier: 'Umsatz', sheetQualifier: 'Analyse' });
+  });
+
+  it('skips function names, strings and named ranges; tolerates unfinished input', () => {
+    expect(texts('=LOG10(A1)&"B2"&Umsatz_Q1')).toEqual(['A1']);
+    expect(texts('=SUMME(A1:A5;B')).toEqual(['A1:A5']);
+    expect(texts('=WENN(A1;"offen')).toEqual([]);
+    expect(texts('A1')).toEqual([]);
+  });
+});