feat: add custom delimiter input and update subtitle text

Delimiter dropdown now includes "Other" option with a text input for
custom delimiter characters. Subtitle updated to mention delimited text.

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

src/gui/app.py

@@ -68,7 +68,7 @@ for key, default in _DEFAULTS.items():
 # ---------------------------------------------------------------------------
 
 st.title("DataTools Deduplicator")
-st.caption("Find and remove duplicate rows in CSV and Excel files.")
+st.caption("Find and remove duplicate rows in CSV, delimited text, and Excel files.")
 
 
 # ---------------------------------------------------------------------------
@@ -151,6 +151,7 @@ if uploaded is not None:
                 "Tab (\\t)": "\t",
                 "Semicolon (;)": ";",
                 "Pipe (|)": "|",
+                "Other": None,
             }
             _DELIM_LABELS = list(_DELIMITERS.keys())
             _DELIM_VALUES = list(_DELIMITERS.values())
@@ -162,7 +163,15 @@ if uploaded is not None:
                 index=default_idx,
                 help="Auto-detected on upload. Change if the preview looks wrong.",
             )
-            chosen_delim = _DELIMITERS[chosen_label]
+            if chosen_label == "Other":
+                custom_delim = st.text_input(
+                    "Enter delimiter character",
+                    max_chars=5,
+                    help="Enter the character(s) used to separate fields.",
+                )
+                chosen_delim = custom_delim if custom_delim else ","
+            else:
+                chosen_delim = _DELIMITERS[chosen_label]
             if chosen_delim != st.session_state.get("_current_delimiter"):
                 st.session_state["_current_delimiter"] = chosen_delim
                 import tempfile