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/pdf_extract.py

@@ -531,23 +531,82 @@ def page_has_extractable_text(page: Page, min_words: int = 5) -> bool:
     return len(page.words) >= min_words
 
 
+def _autodetect_tesseract_path() -> str | None:
+    """Probe well-known install locations for ``tesseract.exe``.
+
+    UB-Mannheim's Windows installer drops Tesseract at one of two
+    paths by default. Auto-detecting them lets ``ocr_available``
+    succeed even when the user (or their installer) skipped the
+    "Add to PATH" step — the most common Windows install
+    snag based on real user reports.
+
+    No-op on non-Windows: macOS/Linux package managers
+    always put ``tesseract`` on PATH, so PATH-based discovery is
+    sufficient.
+    """
+    import os as _os
+    import platform as _platform
+    from pathlib import Path as _Path
+
+    if _platform.system() != "Windows":
+        return None
+    candidates = [
+        r"C:\Program Files\Tesseract-OCR\tesseract.exe",
+        r"C:\Program Files (x86)\Tesseract-OCR\tesseract.exe",
+        _os.path.expandvars(
+            r"%LOCALAPPDATA%\Programs\Tesseract-OCR\tesseract.exe"
+        ),
+    ]
+    for p in candidates:
+        if p and _Path(p).exists():
+            return p
+    return None
+
+
 def ocr_available() -> tuple[bool, str]:
     """Return ``(available, reason)`` — is OCR usable right now?
 
     Checks both the Python binding (``pytesseract``) and the
-    Tesseract binary. The reason string is suitable for surfacing to
-    the user when OCR is unavailable.
+    Tesseract binary. The reason string is suitable for surfacing
+    to the user when OCR is unavailable.
+
+    Discovery order for the Tesseract binary:
+
+    1. ``DATATOOLS_TESSERACT_PATH`` env var — explicit override,
+       wins over everything else. Useful for portable installs.
+    2. Whatever's on PATH (``pytesseract``'s default).
+    3. ``_autodetect_tesseract_path`` — known Windows install
+       locations. Sets ``pytesseract.pytesseract.tesseract_cmd``
+       so subsequent ``image_to_data`` calls use the same binary.
     """
+    import os as _os
+
     try:
-        import pytesseract  # noqa: F401
+        import pytesseract  # noqa: F401, PLC0415
     except ImportError:
         return False, "pytesseract is not installed."
+
+    override = _os.environ.get("DATATOOLS_TESSERACT_PATH")
+    if override:
+        pytesseract.pytesseract.tesseract_cmd = override
+
     try:
-        import pytesseract as pt
-        pt.get_tesseract_version()
-    except Exception as e:
-        return False, f"Tesseract binary not found: {e}"
-    return True, ""
+        pytesseract.get_tesseract_version()
+        return True, ""
+    except Exception as e_path:
+        # Fallback: probe known install locations.
+        candidate = _autodetect_tesseract_path()
+        if candidate:
+            pytesseract.pytesseract.tesseract_cmd = candidate
+            try:
+                pytesseract.get_tesseract_version()
+                return True, ""
+            except Exception as e_candidate:
+                return False, (
+                    f"Tesseract found at {candidate} but failed to "
+                    f"run: {e_candidate}"
+                )
+        return False, f"Tesseract binary not found on PATH: {e_path}"
 
 
 def render_page_image(