fix(pdf): graceful fallback when PDF dependencies aren't installed

User hit a hard ImportError on opening the PDF→CSV tool because
``pip install -r requirements.txt`` hadn't picked up the new
``pdfplumber`` / ``pypdfium2`` lines yet. Streamlit surfaces
that as an unfiltered traceback — friendlier to show a clear
install-required panel inside the tool instead.

Two changes:

1. ``src/pdf_extract.py`` lazy-imports the PDF deps via
   ``_require_pdfplumber()`` / ``_require_pdfium()`` helpers that
   raise a new ``PdfDependencyMissing`` (subclass of ImportError)
   with an actionable ``hint`` field. Pure helpers
   (``parse_amount``, ``parse_date``, ``cluster_rows``, etc.)
   keep working with no PDF dep installed — useful for tests and
   for keeping module-import paths cheap.

2. The tool page probes both deps at render time via
   ``_pdf_deps_status()``; if anything's missing it shows a
   ``st.error`` panel with the exact pip command and a
   "restart the launcher" reminder, then ``st.stop()``s before
   touching any PDF code path.

The page itself loads cleanly without the deps installed, so the
sidebar nav doesn't 500 — the user just sees the install panel
on click.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/gui/pages/10_PDF_Extractor.py

@@ -32,11 +32,26 @@ if str(_project_root) not in sys.path:
 from src.audit import log_event, log_page_open
 from src.gui.components import hide_streamlit_chrome, render_sticky_footer
 from src.pdf_extract import (
+    PdfDependencyMissing,
     apply_template,
     extract_pages_auto,
     ocr_available,
     render_page_image,
 )
+
+
+def _pdf_deps_status() -> tuple[bool, list[str]]:
+    """Probe each runtime PDF dep without forcing the user to hit the
+    extract button. Returns ``(ok, missing_names)``."""
+    missing: list[str] = []
+    for name in ("pdfplumber", "pypdfium2"):
+        try:
+            __import__(name)
+        except ImportError:
+            missing.append(name)
+    return (not missing), missing
+
+
 from src.pdf_templates import (
     SCHEMA_VERSION,
     VALID_TARGETS,
@@ -94,6 +109,19 @@ st.caption(
     "every statement that follows the same layout."
 )
 
+_pdf_ok, _pdf_missing = _pdf_deps_status()
+if not _pdf_ok:
+    st.error(
+        "**PDF dependencies are not installed.** "
+        f"Missing module(s): `{', '.join(_pdf_missing)}`.\n\n"
+        "Install them into the same Python that launches DataTools:\n\n"
+        "```\npip install pdfplumber pypdfium2 "
+        "streamlit-drawable-canvas pytesseract\n```\n\n"
+        "Then **fully restart the launcher** to pick up the new modules. "
+        "(Templates you've already saved are unaffected.)"
+    )
+    st.stop()
+
 _ocr_ok, _ocr_reason = ocr_available()
 c_mode, c_ocr = st.columns([3, 2])
 with c_mode: