feat: add delimiter selector for CSV/TSV files in GUI

Auto-detects delimiter on upload and shows a selectbox with comma, tab,
semicolon, and pipe options. Changing re-reads the file immediately.
Line terminators (Windows/Unix/Mac) already handled by universal newlines.

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

src/gui/app.py

@@ -19,7 +19,7 @@ if str(_project_root) not in sys.path:
     sys.path.insert(0, str(_project_root))
 
 from src.core.dedup import deduplicate, build_default_strategies, DeduplicationResult
-from src.core.io import read_file, list_sheets
+from src.core.io import read_file, list_sheets, detect_encoding, detect_delimiter
 from src.core.config import DeduplicationConfig
 from src.gui.components import (
     apply_review_decisions,
@@ -56,6 +56,7 @@ _DEFAULTS = {
     "config": None,
     "file_name": "",
     "sheet_names": [],
+    "detected_delimiter": ",",
 }
 for key, default in _DEFAULTS.items():
     if key not in st.session_state:
@@ -96,11 +97,14 @@ if uploaded is not None:
                 tmp.write(uploaded.getvalue())
                 tmp_path = Path(tmp.name)
 
-            # Check for Excel sheets
+            # Check for Excel sheets / detect delimiter
             if suffix.lower() in (".xlsx", ".xls"):
                 st.session_state["sheet_names"] = list_sheets(tmp_path)
+                st.session_state["detected_delimiter"] = ","
             else:
                 st.session_state["sheet_names"] = []
+                enc = detect_encoding(tmp_path)
+                st.session_state["detected_delimiter"] = detect_delimiter(tmp_path, enc)
 
             df = read_file(tmp_path)
             if not isinstance(df, pd.DataFrame):
@@ -139,6 +143,41 @@ if uploaded is not None:
                 st.session_state["review_decisions"] = {}
                 tmp_path.unlink(missing_ok=True)
 
+        # Delimiter selector for CSV/TSV files
+        is_csv = Path(uploaded.name).suffix.lower() not in (".xlsx", ".xls")
+        if is_csv:
+            _DELIMITERS = {
+                "Comma (,)": ",",
+                "Tab (\\t)": "\t",
+                "Semicolon (;)": ";",
+                "Pipe (|)": "|",
+            }
+            _DELIM_LABELS = list(_DELIMITERS.keys())
+            _DELIM_VALUES = list(_DELIMITERS.values())
+            detected = st.session_state.get("detected_delimiter", ",")
+            default_idx = _DELIM_VALUES.index(detected) if detected in _DELIM_VALUES else 0
+            chosen_label = st.selectbox(
+                "Delimiter",
+                _DELIM_LABELS,
+                index=default_idx,
+                help="Auto-detected on upload. Change if the preview looks wrong.",
+            )
+            chosen_delim = _DELIMITERS[chosen_label]
+            if chosen_delim != st.session_state.get("_current_delimiter"):
+                st.session_state["_current_delimiter"] = chosen_delim
+                import tempfile
+                suffix = Path(uploaded.name).suffix
+                with tempfile.NamedTemporaryFile(delete=False, suffix=suffix) as tmp:
+                    tmp.write(uploaded.getvalue())
+                    tmp_path = Path(tmp.name)
+                df = read_file(tmp_path, delimiter=chosen_delim)
+                if not isinstance(df, pd.DataFrame):
+                    df = pd.concat(list(df), ignore_index=True)
+                st.session_state["df"] = df
+                st.session_state["result"] = None
+                st.session_state["review_decisions"] = {}
+                tmp_path.unlink(missing_ok=True)
+
         # Preview
         st.subheader(f"Preview: {uploaded.name}")
         st.caption(f"{len(df)} rows, {len(df.columns)} columns")