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/store/__tests__/format.spec.ts

@@ -1,6 +1,6 @@
 import { describe, it, expect } from 'vitest';
 import { dateToSerial, timeSerial } from '../../engine';
-import { formatValue } from '../format';
+import { formatValue, readableTextOn } from '../format';
 
 describe('formatValue: date/time', () => {
   const day = dateToSerial(2026, 9, 2); // a Wednesday
@@ -46,3 +46,23 @@ describe('formatValue: date/time', () => {
     expect(formatValue(day, 'plain', 'dateiso')).toBe('2026-09-02');
   });
 });
+
+describe('readableTextOn', () => {
+  it('light text on dark fills, dark text on light fills', () => {
+    expect(readableTextOn('#000000')).toBe('#f5f5f7');
+    expect(readableTextOn('#000')).toBe('#f5f5f7');
+    expect(readableTextOn('#1b1b1b')).toBe('#f5f5f7');
+    expect(readableTextOn('#0057ff')).toBe('#f5f5f7');
+    expect(readableTextOn('#ffffff')).toBe('#1d1d1f');
+    expect(readableTextOn('#ffd400')).toBe('#1d1d1f');
+    expect(readableTextOn('#FFF')).toBe('#1d1d1f');
+  });
+
+  it('ignores what it cannot read, and mostly transparent fills', () => {
+    expect(readableTextOn(undefined)).toBeNull();
+    expect(readableTextOn('red')).toBeNull();
+    expect(readableTextOn('#12345')).toBeNull();
+    expect(readableTextOn('#00000020')).toBeNull();
+    expect(readableTextOn('#000000cc')).toBe('#f5f5f7');
+  });
+});