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/SettingsPanel.vue

@@ -208,8 +208,9 @@ function resetDoc() {
             </label>
 
             <label class="f row-inline">
-              <button class="tgl" :class="{ on: selFmt.bold }" @click="applyFmt({ bold: !selFmt.bold })"><b>F</b></button>
-              <button class="tgl" :class="{ on: selFmt.italic }" @click="applyFmt({ italic: !selFmt.italic })"><i>K</i></button>
+              <button class="tgl" :class="{ on: selFmt.bold }" title="Fett (Strg+Umschalt+B)" @click="applyFmt({ bold: !selFmt.bold })"><b>F</b></button>
+              <button class="tgl" :class="{ on: selFmt.italic }" title="Kursiv (Strg+Umschalt+I)" @click="applyFmt({ italic: !selFmt.italic })"><i>K</i></button>
+              <button class="tgl" :class="{ on: selFmt.underline }" title="Unterstrichen (Strg+Umschalt+U)" @click="applyFmt({ underline: !selFmt.underline })"><u>U</u></button>
             </label>
 
             <label class="f">