feat: refactor GUI to multi-page Streamlit app with 9 tool pages

Convert single-page deduplicator into a multi-page suite. Home page shows
tool card grid. Deduplicator extracted to its own page (fully working).
8 stub pages added for Text Cleaner, Format Standardizer, Missing Values,
Column Mapper, Outlier Detector, Multi-File Merger, Validator & Reporter,
and Pipeline Runner — each with functional file upload and coming-soon UI.

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

src/gui/pages/7_Multi_File_Merger.py

@@ -0,0 +1,86 @@
+"""DataTools Multi-File Merger — stub page."""
+
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+import streamlit as st
+
+_project_root = Path(__file__).resolve().parent.parent.parent.parent
+if str(_project_root) not in sys.path:
+    sys.path.insert(0, str(_project_root))
+
+# ---------------------------------------------------------------------------
+# Header
+# ---------------------------------------------------------------------------
+
+st.title("📎 Multi-File Merger")
+st.caption("Combine multiple CSV and Excel files into one dataset.")
+
+st.info("This tool is under development.")
+
+# ---------------------------------------------------------------------------
+# What this tool will do
+# ---------------------------------------------------------------------------
+
+st.markdown("""
+**Features:**
+- Upload multiple CSV/Excel files at once
+- Automatic schema alignment (matching columns by name)
+- Append mode: stack files vertically (union)
+- Join mode: merge files on shared key columns
+- Handle mismatched columns (fill missing with nulls or drop)
+- Source file tracking column
+""")
+
+st.divider()
+
+# ---------------------------------------------------------------------------
+# Multi-file upload (functional)
+# ---------------------------------------------------------------------------
+
+uploaded_files = st.file_uploader(
+    "Upload CSV or Excel files",
+    type=["csv", "tsv", "xlsx", "xls"],
+    accept_multiple_files=True,
+    help="Upload multiple files to preview. Processing is not yet available.",
+    key="merger_file_upload",
+)
+
+if uploaded_files:
+    import pandas as pd
+    for f in uploaded_files:
+        try:
+            if f.name.endswith((".xlsx", ".xls")):
+                df = pd.read_excel(f)
+            else:
+                df = pd.read_csv(f)
+            st.subheader(f"Preview: {f.name}")
+            st.caption(f"{len(df)} rows, {len(df.columns)} columns — Columns: {', '.join(df.columns[:10])}{'...' if len(df.columns) > 10 else ''}")
+            st.dataframe(df.head(5), use_container_width=True)
+        except Exception as e:
+            st.error(f"Failed to read {f.name}: {e}")
+
+# ---------------------------------------------------------------------------
+# Placeholder options
+# ---------------------------------------------------------------------------
+
+st.subheader("Merge Strategy")
+
+st.selectbox("Mode", ["Append (stack vertically)", "Join on key columns", "Schema alignment (smart merge)"], disabled=True)
+st.selectbox("Mismatched columns", ["Fill with null", "Drop non-shared columns", "Error"], disabled=True)
+st.checkbox("Add source filename column", value=True, disabled=True)
+
+st.divider()
+st.button("Merge Files", type="primary", use_container_width=True, disabled=True)
+
+# ---------------------------------------------------------------------------
+# Footer
+# ---------------------------------------------------------------------------
+
+st.divider()
+st.caption(
+    "Runs locally. Your data never leaves this computer. "
+    "| DataTools v3.0"
+)