feat(pdf): robust Tesseract discovery + OS-aware install copy

User tried ``brew install tesseract`` in PowerShell after seeing
all three OSes listed inline in the OCR banner — easy mistake
when the install commands are crammed on one line with ``·``
separators. Two changes pre-empt this:

**OS-aware OCR banner.** The expander now detects the user's
platform via ``platform.system()`` and shows only the relevant
install instructions:

- **Windows**: UB-Mannheim installer link, numbered steps,
  explicit "keep the Add to PATH checkbox on" callout, plus a
  fallback paragraph telling the user how to set
  ``DATATOOLS_TESSERACT_PATH`` if they already installed
  without PATH and don't want to reinstall.
- **macOS**: ``brew install tesseract`` with a Homebrew link.
- **Linux**: ``apt install tesseract-ocr`` with a "or your
  distro's equivalent" hedge.

**Robust binary discovery in ``ocr_available()``.** Three-stage:

1. Honor ``DATATOOLS_TESSERACT_PATH`` env var if set — explicit
   override for portable installs or non-default locations.
2. Try ``pytesseract``'s default PATH-based lookup.
3. If PATH lookup fails, probe known Windows install paths
   (``C:\Program Files\Tesseract-OCR\tesseract.exe``,
   the x86 variant, and ``%LOCALAPPDATA%\Programs\Tesseract-OCR\``)
   via the new ``_autodetect_tesseract_path``. On hit, set
   ``pytesseract.pytesseract.tesseract_cmd`` so all subsequent
   ``image_to_data`` calls use the same binary without
   re-discovering.

This means a user who runs the UB-Mannheim installer with
default options but forgets the PATH checkbox will still get
OCR working after a launcher restart, without env-var
gymnastics.

Tests (4 new, 85 total in the suite):

- Auto-detect returns None on non-Windows (no false positives
  on dev laptops).
- Auto-detect finds the binary at a mocked
  ``C:\Program Files\Tesseract-OCR\tesseract.exe``.
- Auto-detect returns None when no candidate exists.
- ``DATATOOLS_TESSERACT_PATH`` env var beats both PATH lookup
  and auto-detect (sets ``tesseract_cmd`` even when the path
  doesn't resolve, so a real binary at a custom location works).

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

src/gui/pages/10_PDF_Extractor.py

@@ -136,16 +136,44 @@ with c_ocr:
     if _ocr_ok:
         st.caption("**OCR:** ready · scanned pages will be transcribed.")
     else:
+        import platform as _platform
+        _os_name = _platform.system()
         with st.expander("**OCR:** unavailable", expanded=False):
-            st.caption(
-                f"Reason: {_ocr_reason or 'unknown'}. Scanned (image-based) "
-                "statements will fall through with warnings. "
-                "To enable OCR, install Tesseract on this machine — "
-                "[Windows](https://github.com/UB-Mannheim/tesseract/wiki) · "
-                "macOS: ``brew install tesseract`` · "
-                "Linux: ``apt install tesseract-ocr``. "
-                "Modern text-based statements don't need OCR."
+            st.markdown(
+                f"**Reason:** {_ocr_reason or 'unknown'}.\n\n"
+                "Scanned (image-based) statements will fall through "
+                "with warnings. Most modern bank statements are text-"
+                "based and don't need OCR — only install Tesseract if "
+                "your statements actually come through as images."
             )
+            if _os_name == "Windows":
+                st.markdown(
+                    "**Install on Windows:**\n"
+                    "1. Download the installer from "
+                    "[UB-Mannheim/tesseract](https://github.com/UB-Mannheim/tesseract/wiki) "
+                    "(look for ``tesseract-ocr-w64-setup-…``).\n"
+                    "2. Run it. Keep the **\"Add tesseract to system "
+                    "PATH\"** checkbox on during setup.\n"
+                    "3. Restart the DataTools launcher.\n\n"
+                    "If you installed without PATH and don't want to "
+                    "reinstall, point DataTools at the binary directly "
+                    "by setting the ``DATATOOLS_TESSERACT_PATH`` env "
+                    "var to ``C:\\Program Files\\Tesseract-OCR\\tesseract.exe`` "
+                    "before launching."
+                )
+            elif _os_name == "Darwin":
+                st.markdown(
+                    "**Install on macOS:** ``brew install tesseract`` "
+                    "(requires [Homebrew](https://brew.sh)). Restart "
+                    "the DataTools launcher afterward."
+                )
+            else:
+                st.markdown(
+                    "**Install on Linux:** ``sudo apt install "
+                    "tesseract-ocr`` (Debian/Ubuntu) or your distro's "
+                    "equivalent (``dnf``, ``pacman``, …). Restart the "
+                    "DataTools launcher afterward."
+                )
 
 st.divider()