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/index.ts

@@ -8,6 +8,7 @@
 export { tokenize, LexError, type Token, type TokenType } from './lexer';
 export {
   parseFormula,
+  completeFormula,
   parseFunctionBlock,
   parseMacroBlock,
   collectRefs,
@@ -37,6 +38,8 @@ export {
   WHOLE_LINE_END,
   cycleReferenceLocks,
   currentIdentifierAtCaret,
+  formulaRefSpans,
+  type RefSpan,
 } from './refs';
 export {
   Workbook,