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/io/json.ts

@@ -171,6 +171,7 @@ function normaliseCellFormat(x: unknown): CellFormat | undefined {
   if (colorOrUndef(f.bg)) out.bg = f.bg;
   if (f.bold === true) out.bold = true;
   if (f.italic === true) out.italic = true;
+  if (f.underline === true) out.underline = true;
   return Object.keys(out).length ? out : undefined;
 }
 function normaliseFormats(x: unknown): Record<string, CellFormat> | undefined {