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/components/FormulaBar.vue

@@ -1,7 +1,7 @@
 <script setup lang="ts">
 import { computed, nextTick, ref, watch } from 'vue';
 import { useDocument } from '../store/document';
-import { cycleReferenceLocks } from '../engine';
+import { cycleReferenceLocks, completeFormula } from '../engine';
 import { useFormulaAutocomplete } from '../composables/useFormulaAutocomplete';
 import { useKeyboardPointMode } from '../composables/useKeyboardPointMode';
 import { useToasts } from '../composables/useToasts';
@@ -113,6 +113,10 @@ function backToGrid() {
 
 function commit() {
   const sel = store.doc.selection;
+  // a forgotten closing ")" is completed rather than stored as an error; in
+  // the draft too, since the sync watch above skips changes made while the
+  // field still has focus (Enter commits before it blurs)
+  draft.value = completeFormula(draft.value);
   if (sel) store.setCellInput(sel.tableId, sel.anchor.col, sel.anchor.row, draft.value);
 }
 /** Lets the store insert a reference into this field when the user clicks a