feat: initial Dropbox tag sync tool

Cross-platform CLI that mirrors Dropbox web tags to OS-native tags:
- macOS: Finder xattrs (content-preserving, no re-upload)
- Windows: XMP:Subject via ExifTool (re-upload expected once per file)

Includes an interactive wizard that auto-detects the OS and Dropbox
folder, walks the user through read-only token creation, previews
tagged files, applies tags, and installs a background scheduler
(launchd on macOS, Task Scheduler on Windows).

Ships with:
- 47-test suite (pytest) covering config, state DB, run_sync, writers
- Gitea Actions CI for Linux / macOS / Windows with coverage reporting

tests/test_platform_windows.py

@@ -0,0 +1,154 @@
+"""Tests for tagsync.platform_windows: sidecar writer + support matrix.
+
+Runs on any OS — the Windows-specific online-only stat test is skipped
+except on Windows.
+"""
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+
+from tagsync.platform_windows import (
+    XMP_WRITABLE_EXTS,
+    WindowsWriter,
+)
+
+
+# --- supports() --------------------------------------------------------------
+
+
+def test_supports_xmp_formats():
+    w = WindowsWriter(exiftool="exiftool", write_sidecars=False)
+    for ext in (".jpg", ".pdf", ".docx", ".mp4"):
+        assert w.supports(Path(f"foo{ext}")), f"expected support for {ext}"
+
+
+def test_supports_rejects_unknown_without_sidecars():
+    w = WindowsWriter(exiftool="exiftool", write_sidecars=False)
+    assert not w.supports(Path("archive.zip"))
+    assert not w.supports(Path("notes.txt"))
+
+
+def test_supports_accepts_unknown_with_sidecars():
+    w = WindowsWriter(exiftool="exiftool", write_sidecars=True)
+    assert w.supports(Path("archive.zip"))
+    assert w.supports(Path("notes.txt"))
+
+
+def test_supports_is_case_insensitive():
+    w = WindowsWriter(exiftool="exiftool", write_sidecars=False)
+    assert w.supports(Path("IMAGE.JPG"))
+    assert w.supports(Path("report.PDF"))
+
+
+# --- sidecar writer ---------------------------------------------------------
+
+
+def test_sidecar_written_for_unsupported_type(tmp_path):
+    w = WindowsWriter(exiftool="exiftool", write_sidecars=True)
+    target = tmp_path / "archive.zip"
+    target.touch()
+
+    w.write_tags(target, ["project-alpha", "urgent"])
+
+    sidecar = tmp_path / "archive.zip.tags.json"
+    assert sidecar.exists()
+    data = json.loads(sidecar.read_text())
+    assert data == {"tags": ["project-alpha", "urgent"]}
+
+
+def test_sidecar_cleared_when_tags_empty(tmp_path):
+    w = WindowsWriter(exiftool="exiftool", write_sidecars=True)
+    target = tmp_path / "archive.zip"
+    target.touch()
+    (tmp_path / "archive.zip.tags.json").write_text('{"tags": ["x"]}')
+
+    w.write_tags(target, [])
+    assert not (tmp_path / "archive.zip.tags.json").exists()
+
+
+def test_sidecar_clear_nonexistent_is_ok(tmp_path):
+    w = WindowsWriter(exiftool="exiftool", write_sidecars=True)
+    target = tmp_path / "archive.zip"
+    target.touch()
+    # Should not raise even though no sidecar exists.
+    w.write_tags(target, [])
+
+
+def test_unsupported_without_sidecars_raises(tmp_path):
+    w = WindowsWriter(exiftool="exiftool", write_sidecars=False)
+    target = tmp_path / "archive.zip"
+    target.touch()
+    with pytest.raises(RuntimeError):
+        w.write_tags(target, ["tag"])
+
+
+# --- exiftool invocation ----------------------------------------------------
+#
+# We don't actually run exiftool; monkeypatch subprocess.run to capture args.
+
+
+def test_exiftool_args_are_well_formed(tmp_path, monkeypatch):
+    captured = {}
+
+    def fake_run(args, **kwargs):
+        captured["args"] = args
+        class R:
+            returncode = 0
+            stderr = ""
+        return R()
+
+    import tagsync.platform_windows as pw
+    monkeypatch.setattr(pw.subprocess, "run", fake_run)
+
+    w = WindowsWriter(exiftool="exiftool", write_sidecars=False)
+    target = tmp_path / "pic.jpg"
+    target.touch()
+    w.write_tags(target, ["alpha", "beta"])
+
+    args = captured["args"]
+    assert args[0] == "exiftool"
+    assert "-overwrite_original" in args
+    # Subject should be cleared then appended for each tag.
+    assert "-XMP-dc:Subject=" in args
+    assert "-XMP-dc:Subject+=alpha" in args
+    assert "-XMP-dc:Subject+=beta" in args
+    assert str(target) in args
+
+
+def test_exiftool_failure_raises(tmp_path, monkeypatch):
+    def fake_run(args, **kwargs):
+        class R:
+            returncode = 1
+            stderr = "bad things happened"
+        return R()
+
+    import tagsync.platform_windows as pw
+    monkeypatch.setattr(pw.subprocess, "run", fake_run)
+
+    w = WindowsWriter(exiftool="exiftool", write_sidecars=False)
+    target = tmp_path / "pic.jpg"
+    target.touch()
+    with pytest.raises(RuntimeError, match="bad things happened"):
+        w.write_tags(target, ["x"])
+
+
+# --- online-only detection --------------------------------------------------
+
+
+@pytest.mark.windows
+def test_is_online_only_returns_false_for_normal_files(tmp_path):
+    w = WindowsWriter(exiftool="exiftool")
+    f = tmp_path / "regular.txt"
+    f.touch()
+    assert w.is_online_only(f) is False
+
+
+def test_is_online_only_returns_false_when_attribute_unavailable(tmp_path):
+    """On non-Windows, st_file_attributes doesn't exist -> should return False."""
+    w = WindowsWriter(exiftool="exiftool")
+    f = tmp_path / "regular.txt"
+    f.touch()
+    assert w.is_online_only(f) is False