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.

label_config.py

@@ -0,0 +1,43 @@
+"""
+Persistence for the PT-E560BT label layout settings.
+
+Stored as JSON next to the application so the operator's logo, static text and
+QR template survive restarts. Kept deliberately tolerant: a missing or corrupt
+file silently yields defaults rather than blocking printing.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+
+from ptouch_label import LabelConfig
+
+CONFIG_FILENAME = "label_config.json"
+
+
+def config_path(base_dir: str | None = None) -> str:
+    """Absolute path to the settings file."""
+    directory = base_dir or os.path.dirname(os.path.abspath(__file__))
+    return os.path.join(directory, CONFIG_FILENAME)
+
+
+def load_config(base_dir: str | None = None) -> LabelConfig:
+    """Load settings, falling back to defaults when absent or unreadable."""
+    path = config_path(base_dir)
+    if not os.path.exists(path):
+        return LabelConfig()
+    try:
+        with open(path, "r", encoding="utf-8") as handle:
+            return LabelConfig.from_dict(json.load(handle))
+    except (OSError, ValueError) as exc:
+        print(f"Could not read {CONFIG_FILENAME} ({exc}); using defaults.")
+        return LabelConfig()
+
+
+def save_config(config: LabelConfig, base_dir: str | None = None) -> str:
+    """Write settings to disk and return the path written."""
+    path = config_path(base_dir)
+    with open(path, "w", encoding="utf-8") as handle:
+        json.dump(config.to_dict(), handle, indent=2, ensure_ascii=False)
+    return path