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/3_Format_Standardizer.py

@@ -0,0 +1,86 @@
+"""DataTools Format Standardizer — 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("📐 Format Standardizer")
+st.caption("Standardize formats across columns for consistency.")
+
+st.info("This tool is under development.")
+
+# ---------------------------------------------------------------------------
+# What this tool will do
+# ---------------------------------------------------------------------------
+
+st.markdown("""
+**Features:**
+- Date format standardization (e.g., MM/DD/YYYY → YYYY-MM-DD)
+- Phone number formatting (E.164, national, international)
+- Currency normalization ($1,000.00 → 1000.00)
+- Name casing (JOHN DOE → John Doe)
+- Address abbreviation expansion (St. → Street, Ave. → Avenue)
+- Boolean standardization (Yes/No/Y/N/1/0 → True/False)
+""")
+
+st.divider()
+
+# ---------------------------------------------------------------------------
+# File upload (functional)
+# ---------------------------------------------------------------------------
+
+uploaded = st.file_uploader(
+    "Upload CSV or Excel file",
+    type=["csv", "tsv", "xlsx", "xls"],
+    help="Upload a file to preview. Processing is not yet available.",
+    key="fmtstd_file_upload",
+)
+
+if uploaded is not None:
+    import pandas as pd
+    try:
+        if uploaded.name.endswith((".xlsx", ".xls")):
+            df = pd.read_excel(uploaded)
+        else:
+            df = pd.read_csv(uploaded)
+        st.subheader(f"Preview: {uploaded.name}")
+        st.caption(f"{len(df)} rows, {len(df.columns)} columns")
+        st.dataframe(df.head(10), use_container_width=True)
+    except Exception as e:
+        st.error(f"Failed to read file: {e}")
+
+# ---------------------------------------------------------------------------
+# Placeholder options
+# ---------------------------------------------------------------------------
+
+st.subheader("Format Rules")
+
+st.selectbox("Date format", ["YYYY-MM-DD", "MM/DD/YYYY", "DD/MM/YYYY", "DD-Mon-YYYY"], disabled=True)
+st.selectbox("Phone format", ["E.164 (+15551234567)", "National ((555) 123-4567)", "Digits only"], disabled=True)
+st.selectbox("Currency handling", ["Strip symbols, keep number", "Normalize to 2 decimals", "Keep as-is"], disabled=True)
+st.selectbox("Name casing", ["Title Case", "UPPER", "lower", "As-is"], disabled=True)
+st.checkbox("Expand address abbreviations", value=False, disabled=True)
+
+st.divider()
+st.button("Standardize Formats", type="primary", use_container_width=True, disabled=True)
+
+# ---------------------------------------------------------------------------
+# Footer
+# ---------------------------------------------------------------------------
+
+st.divider()
+st.caption(
+    "Runs locally. Your data never leaves this computer. "
+    "| DataTools v3.0"
+)