Stempel: Stile + Layout-Customisation (Header, Show-Toggles)

Bilanz-Stempel hat jetzt:
- Custom Header (Default "Nutzflächen", frei editierbar)
- Per-Stempel Show-Toggles: scope, hnf, nnf, nf, vf, ff, ngf, gf, agf,
  count (Anzahl Raeume), separators (Trennlinien)
- Stempel-Stile (Presets) per Doc — separate Storage von Raumstempel-
  Stilen (dossier_stempel_stile)

Backend (elemente.py):
- UserStrings: dossier_stempel_header + dossier_stempel_show_* (11 Flags)
  + dossier_stempel_stil_id
- _make_stempel_text erweitert: header_text, show_scope, show_separators,
  show_count, visibility-dict
- _format_bilanz_lines: visibility + show_separators + show_count Args
- load_stempel_stile / save_stempel_stile (eigener doc.Strings-Key)
- Bridge-Handler SAVE/DELETE/APPLY_STEMPEL_STIL (analog Raum-Stil-Pattern,
  inkl. Bulk-Apply via Selection + applyToIds-Mechanism wie SaveRaum-Stil)
- _sync_stempel_to_source spiegelt Font/Bold/Italic/TextHeight vom Live-
  TextEntity zur UserString-Storage (Oberleiste-Edits ueberleben Regen)
- _update_wall stempel-Branch um header + show-Flags erweitert

Frontend (StempelProperties):
- Stil-Picker-Dropdown analog Raum (Stil wählen / + speichern / 🗑 löschen)
- Header-Input (Inline-Text)
- 11 Show-Toggles als kompakte Grid (Check-Box-Stil)
- Live-Vorschau respektiert Visibility-Flags + bilanz.count

src/lib/rhinoBridge.js

@@ -256,6 +256,15 @@ export function reorderRaumStile(ids) { send('REORDER_RAUM_STILE', { ids }) }
 export function duplicateRaumStil(id, newName) {
   send('DUPLICATE_RAUM_STIL', { id, newName })
 }
+// Stempel-Stile (analog Raumstempel-Stile, eigene Storage pro Doc)
+export function saveStempelStil(id, name, settings, applyToIds) {
+  send('SAVE_STEMPEL_STIL', { id, name, settings,
+                                applyToIds: applyToIds || [] })
+}
+export function deleteStempelStil(id) { send('DELETE_STEMPEL_STIL', { id }) }
+export function applyStempelStil(stilId, ids) {
+  send('APPLY_STEMPEL_STIL', { stilId, ids })
+}
 export function setSectionStyle(enabled, source, color, pattern, scale, rotation,
                                 opts = {}) {
   send('SET_SECTION_STYLE', {