Fix text area reset after AI revision, add API key error handling

- Pop draft_editor/quality_editor widget keys before rerun so revised
  text is correctly displayed after AI enrichment calls
- Wrap _get_client() with friendly st.error/st.stop instead of stack trace
- Remove unused _back_next() helper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -11,7 +11,11 @@ import streamlit as st
 @st.cache_resource
 def _get_client():
     from kniepunkt.llm import get_client
-    return get_client()
+    try:
+        return get_client()
+    except RuntimeError as e:
+        st.error(f"{e}\n\nBitte ANTHROPIC_API_KEY in der .env-Datei eintragen und Streamlit neu starten.")
+        st.stop()
 
 
 @st.cache_resource
@@ -70,16 +74,6 @@ def _progress_bar():
     st.divider()
 
 
-def _back_next(back_step: str, next_label: str, next_key: str, next_disabled: bool = False):
-    col1, col2 = st.columns([1, 5])
-    with col1:
-        if st.button("← Zurück", key=f"back_{back_step}"):
-            _go(back_step)
-    with col2:
-        if st.button(next_label, type="primary", key=next_key, disabled=next_disabled):
-            return True
-    return False
-
 
 def _grid_2x2(results: dict, select_key: str, button_label: str = "Auswählen"):
     """Render provider outputs in a responsive 2-column grid with selection."""
@@ -345,6 +339,7 @@ def screen_draft():
                     draft = dr_module.enrich_draft(_get_client(), draft, feedback)
                 st.session_state.draft_text = draft
                 st.session_state.draft_text_source = selected
+                st.session_state.pop("draft_editor", None)
                 st.rerun()
 
         st.divider()
@@ -408,6 +403,7 @@ def screen_quality():
                 draft = dr_module.enrich_draft(_get_client(), draft, feedback)
             st.session_state.session["draft"] = draft
             st.session_state.pop("quality_report", None)
+            st.session_state.pop("quality_editor", None)
             _save()
             st.rerun()