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/AppToolbar.vue

@@ -4,6 +4,7 @@ import { useDocument } from '../store/document';
 import { useTheme } from '../composables/useTheme';
 import { useToasts } from '../composables/useToasts';
 import BaseModal from './BaseModal.vue';
+import HelpModal from './HelpModal.vue';
 import CustomFunctionsModal from './CustomFunctionsModal.vue';
 import NamedRangesModal from './NamedRangesModal.vue';
 import MacrosModal from './MacrosModal.vue';
@@ -120,6 +121,17 @@ const uploadOpen = ref(false);
 const loginOpen = ref(false);
 const loginReason = ref<string | undefined>();
 const accountMenuOpen = ref(false);
+const helpOpen = ref(false);
+
+// F1 opens the help from anywhere (also out of a cell editor -- the browser's
+// own F1 help is what preventDefault keeps away)
+function onHelpKey(e: KeyboardEvent) {
+  if (e.key !== 'F1') return;
+  e.preventDefault();
+  helpOpen.value = true;
+}
+onMounted(() => window.addEventListener('keydown', onHelpKey));
+onBeforeUnmount(() => window.removeEventListener('keydown', onHelpKey));
 
 function openLogin(reason?: string) {
   loginReason.value = reason;
@@ -470,10 +482,13 @@ async function onSample() {
         <li><button @click="runXlsxExport">Export · Arbeitsmappe (.xlsx)</button></li>
         <li><button @click="exportSelectedChartPng">Export · gewähltes Diagramm (.png)</button></li>
         <li class="sep" />
+        <li><button @click="helpOpen = true">Hilfe … (F1)</button></li>
         <li><button @click="aboutOpen = true">Über Numolus …</button></li>
       </ul>
     </div>
 
+    <button class="ibtn help" title="Hilfe (F1)" aria-label="Hilfe" @click="helpOpen = true">?</button>
+
     <button
       class="ibtn"
       :title="`Theme: ${themeLabel} (klicken zum Wechseln)`"
@@ -588,6 +603,7 @@ async function onSample() {
   />
   <LoginModal :open="loginOpen" :reason="loginReason" @close="loginOpen = false" />
   <UploadModal :open="uploadOpen" @close="uploadOpen = false" />
+  <HelpModal :open="helpOpen" @close="helpOpen = false" />
 
   <!-- Über / Impressum -->
   <BaseModal :open="aboutOpen" title="Über Numolus" @close="aboutOpen = false">
@@ -866,6 +882,11 @@ async function onSample() {
   cursor: pointer;
   transition: background-color 0.12s ease, color 0.12s ease;
 }
+/* the help button is a plain "?" rather than an icon */
+.ibtn.help {
+  font-size: 15px;
+  font-weight: 700;
+}
 .tgroup.accent .ibtn {
   color: var(--accent);
 }