Add P-touch Labels for Asset: 63x20 mm label format for PT-E560BT

New three-column label format for Brother P-touch EDGE PT-E560BT asset tags:
logo | three static lines + variable field | QR code.

Layout and rendering
- ptouch_label.py: geometry engine with PDF and preview renderers. Both
  consume the same geometry, so the preview matches the print exactly.
- Code symbols are snapped to whole printer dots per module. Fractional
  modules are the usual reason a small QR code will not scan.
- QR is maximised to the printable height by default (16.4 mm, 4 dots per
  module) instead of a fixed 16 mm box (12.3 mm, 3 dots per module).
- Code payload is composed as prefix + inventory + suffix. A legacy
  {inventory} template is migrated on first save.

Hardware handling
- print_area.py reads the driver's real printable area via GetDeviceCaps.
  The PT-E560BT head is 128 dots at 180 dpi, so the maximum print height is
  18.06 mm even on 24 mm tape; the driver reports media as 65x24 mm but only
  about 61x18 mm is reachable. Content is fitted to the reachable area,
  otherwise the driver silently crops it.
- set_printer_resolution.py reads/switches the driver's resolution mode. The
  driver offers 180x180 (symmetric) and 180x360 (anisotropic, 2x across the
  tape); the anisotropic mode scaled output incorrectly.

GUI
- label_printer_ptouch_gui.py: 600x300 two-column panel - controls on the
  left, live preview on the right. Advanced settings for logo, static lines,
  code prefix/suffix and error correction.
- native_file_dialog.py: the standard Windows file picker, run in a
  subprocess because Tk and Kivy both want the main event loop.

Printing
- Print failures are reported honestly. print_to_printer previously returned
  True unconditionally, so a failed print claimed success and was logged as
  SUCCESS.
- SumatraPDF exit codes are now checked (4 = no printer, 5 = driver error).
- The PDF is sent with noscale so driver-side scaling cannot distort the QR
  module pitch.

Tests
- validate_ptouch_layout.py decodes the QR back out of the rendered preview
  module by module, proving the preview and PDF are the same layout.
- test_ptouch_pipeline.py writes to a temp config so it cannot overwrite
  operator settings.
- test_gui_layout.py exercises the real app class headlessly.
- test_print_reporting.py covers the failure paths.

Diagnostics: check_print_area.py, check_print_geometry.py, check_devmode.py,
print_calibration.py, print_measure_scale.py, capture_print_job.py.

native_file_dialog.py

@@ -0,0 +1,106 @@
+"""
+Native Windows file/folder picker, used by the Kivy GUI.
+
+Kivy's built-in FileChooserListView has no address bar, no Quick Access and
+awkward keyboard navigation, which makes finding a file on a real PC painful.
+This shells out to the standard Windows dialog instead.
+
+It runs in a **separate process** on purpose: Tk wants to own the main event
+loop, and Kivy's SDL2 loop is already running in the parent. Isolating Tk in a
+child process avoids that conflict entirely, and the result comes back on stdout.
+
+Usage:
+    python native_file_dialog.py [--dir START_DIR] [--save] [--folder] [--title T]
+
+Output (one line on stdout):
+    SELECTED:<abs path>   on success
+    CANCELLED             if the user dismissed the dialog
+"""
+
+from __future__ import annotations
+
+import argparse
+import os
+import sys
+
+IMAGE_TYPES = [
+    ("Image files", "*.png;*.jpg;*.jpeg;*.bmp;*.gif;*.tif;*.tiff;*.webp"),
+    ("PNG", "*.png"),
+    ("JPEG", "*.jpg;*.jpeg"),
+    ("Bitmap", "*.bmp"),
+    ("All files", "*.*"),
+]
+
+
+def _default_dir(start: str | None) -> str:
+    """Pick a sensible starting folder."""
+    if start and os.path.isdir(start):
+        return start
+    if start and os.path.isfile(start):
+        return os.path.dirname(start)
+
+    # Prefer the user's Pictures folder, then their home directory.
+    home = os.path.expanduser("~")
+    pictures = os.path.join(home, "Pictures")
+    if os.path.isdir(pictures):
+        return pictures
+    return home if os.path.isdir(home) else os.getcwd()
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(description="Native file picker")
+    parser.add_argument("--dir", default=None, help="initial directory or file")
+    parser.add_argument("--title", default="Select a file")
+    parser.add_argument(
+        "--folder", action="store_true", help="pick a folder instead of a file"
+    )
+    parser.add_argument(
+        "--save", action="store_true", help="use the save-as dialog"
+    )
+    args = parser.parse_args()
+
+    try:
+        import tkinter
+        from tkinter import filedialog
+    except ImportError:
+        print("ERROR:tkinter is not available in this Python installation", file=sys.stderr)
+        return 2
+
+    root = tkinter.Tk()
+    root.withdraw()
+    # Keep the dialog on top of the Kivy window rather than behind it.
+    root.attributes("-topmost", True)
+    root.update()
+
+    start = _default_dir(args.dir)
+    try:
+        if args.folder:
+            result = filedialog.askdirectory(
+                title=args.title, initialdir=start, mustexist=True
+            )
+        elif args.save:
+            result = filedialog.asksaveasfilename(
+                title=args.title,
+                initialdir=start,
+                defaultextension=".png",
+                filetypes=IMAGE_TYPES,
+            )
+        else:
+            result = filedialog.askopenfilename(
+                title=args.title, initialdir=start, filetypes=IMAGE_TYPES
+            )
+    finally:
+        try:
+            root.destroy()
+        except tkinter.TclError:
+            pass
+
+    if result:
+        print(f"SELECTED:{os.path.normpath(result)}")
+    else:
+        print("CANCELLED")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())