feat(ui): group validation fields into tabs (Basis/Parteien/Positionen/PDF-Text)

Reduces scrolling - Summen und Export bleiben unterhalb der Tabs immer sichtbar.
Reine UI-Umgruppierung, keine Aenderung an Feldern oder Berechnungslogik.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

main.py

@@ -109,7 +109,6 @@ with col1:
         st.success("PDF eingelesen!")
         uploaded_file.seek(0)
         st.pdf(uploaded_file)
-        with st.expander("Gefundener Text"): st.text(pdf_text)
 
 with col2:
     st.subheader("2. Strukturierte Daten validieren")
@@ -202,6 +201,13 @@ with col2:
         if data.get("hardware_serials"):
             st.caption(f"🔧 Erkannte Seriennummern: {', '.join(data['hardware_serials'])}")
 
+        # Reiter-Navigation: gruppiert die Eingabefelder, damit man nicht die ganze Seite
+        # runterscrollen muss. Summen & Export stehen bewusst UNTERHALB der Tabs (immer sichtbar).
+        tab_basis, tab_parteien, tab_positionen, tab_pdftext = st.tabs(
+            ["Basis", "Parteien", "Positionen", "PDF-Text"]
+        )
+
+        with tab_basis:
             # --- BT-3 RECHNUNGSTYP DROPDOWN-MENÜ ---
             inv_type_ui = st.selectbox(
                 "Rechnungstyp (BT-3)",
@@ -247,10 +253,18 @@ with col2:
                 value=str(data.get("payment_term_days", "") or "")
             )
 
-        i_note = st.text_area("Globale Rechnungserläuterung / Freitext (BT-22)", value=data.get("invoice_note", ""), height=150)
+            payment_means_codes_list = list(PAYMENT_MEANS_CODES.keys())
-        st.markdown("---")
+            payment_means_labels = [f"{code} - {PAYMENT_MEANS_CODES[code]}" for code in payment_means_codes_list]
+            selected_payment_means_label = st.selectbox(
+                "Zahlungsmittel (BT-81)",
+                options=payment_means_labels,
+                index=payment_means_codes_list.index("58"),
+            )
+            selected_payment_means_code = selected_payment_means_label.split(" - ")[0]
 
-        # Adressdaten
+            i_note = st.text_area("Globale Rechnungserläuterung / Freitext (BT-22)", value=data.get("invoice_note", ""), height=150)
+
+        with tab_parteien:
             c_s, c_r = st.columns(2)
             with c_s:
                 st.markdown("**Absender (Verkäufer):**")
@@ -271,19 +285,7 @@ with col2:
                 r_zip = st.text_input("PLZ (BT-53)", value=data.get("recipient_zip", ""))
                 r_city = st.text_input("Stadt (BT-52) :red[*]", value=data.get("recipient_city", ""))
 
-        st.markdown("---")
+        with tab_positionen:
-
-        payment_means_codes_list = list(PAYMENT_MEANS_CODES.keys())
-        payment_means_labels = [f"{code} - {PAYMENT_MEANS_CODES[code]}" for code in payment_means_codes_list]
-        selected_payment_means_label = st.selectbox(
-            "Zahlungsmittel (BT-81)",
-            options=payment_means_labels,
-            index=payment_means_codes_list.index("58"),
-        )
-        selected_payment_means_code = selected_payment_means_label.split(" - ")[0]
-
-        st.markdown("### 3. Positionen")
-        
             if st.button("➕ Neue Position hinzufügen"):
                 data["items"].append({
                     "name": "Neue Dienstleistung", "description": "", "quantity": 1.0, "unit": "Stk.",
@@ -357,6 +359,12 @@ with col2:
             with c_disc4:
                 skonto_days = st.text_input("Skonto-Frist (Tage)", value="")
 
+        with tab_pdftext:
+            st.text(pdf_text if pdf_text.strip() else "(Kein PDF-Text verfügbar)")
+
+        # --- SUMMEN & EXPORT: bewusst AUSSERHALB der Tabs, damit sie unabhängig vom aktiven
+        #     Reiter immer sichtbar und ohne Scrollen erreichbar sind. ---
+        st.markdown("---")
         totals = compute_invoice_totals(edited_items, discount_percent=discount_percent)
         computed_net_amount = totals["tax_basis_total"]
         computed_vat_amount = totals["total_vat_amount"]