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/conftest.py

@@ -0,0 +1,122 @@
+"""Shared pytest fixtures and helpers."""
+from __future__ import annotations
+
+import sys
+from dataclasses import dataclass
+from pathlib import Path
+
+import pytest
+
+
+# --- Isolate persistent state paths from the real user's home ---------------
+
+
+@pytest.fixture(autouse=True)
+def _isolate_user_state(tmp_path, monkeypatch):
+    """Redirect all per-user state files into a tmp dir.
+
+    Ensures tests never clobber real settings, tokens, or state DB on the
+    developer's machine.
+    """
+    fake_home = tmp_path / "home"
+    fake_home.mkdir()
+    monkeypatch.setattr(Path, "home", staticmethod(lambda: fake_home))
+
+    # Re-import config so its module-level paths re-evaluate against the
+    # patched home. We clear and re-import to reset cached module constants.
+    import importlib
+    from tagsync import config
+
+    importlib.reload(config)
+    yield
+    # Leave things as-is after the test; tmp_path cleanup handles disk.
+
+
+# --- Fake Dropbox API objects ------------------------------------------------
+#
+# The Dropbox SDK may or may not be installed in the test env. We don't want
+# to require it, and the parts of core.py that need it are only reached via
+# helpers we can monkeypatch out.
+
+
+@dataclass
+class FakeFileMetadata:
+    """Stand-in for dropbox.files.FileMetadata."""
+    name: str
+    path_lower: str
+    path_display: str
+
+
+@dataclass
+class FakeUserTag:
+    tag_text: str
+
+
+class FakeTag:
+    """Stand-in for dropbox.files.Tag (union type)."""
+    def __init__(self, tag_text: str) -> None:
+        self._user = FakeUserTag(tag_text)
+
+    def get_user_generated_tag(self) -> FakeUserTag:
+        return self._user
+
+
+@dataclass
+class FakePathToTags:
+    path: str
+    tags: list
+
+
+@dataclass
+class FakeTagsResult:
+    paths_to_tags: list
+
+
+@pytest.fixture
+def patched_core(monkeypatch):
+    """Monkeypatch core's Dropbox-dependent helpers to controllable fakes.
+
+    Tests receive a `state` dict where they can configure:
+      - `files`: list of FakeFileMetadata returned by _list_all_files
+      - `tags_by_path`: dict[path_lower -> list[str]] returned by _fetch_tags
+    """
+    from tagsync import core
+
+    state = {
+        "files": [],
+        "tags_by_path": {},
+        "fetch_calls": 0,
+    }
+
+    def fake_client(_token):
+        return object()  # placeholder; never used because list/fetch are faked
+
+    def fake_list(_dbx):
+        return list(state["files"])
+
+    def fake_fetch(_dbx, files):
+        state["fetch_calls"] += 1
+        for f in files:
+            tag_names = sorted(state["tags_by_path"].get(f.path_lower, []))
+            yield f, tag_names
+
+    monkeypatch.setattr(core, "_dropbox_client", fake_client)
+    monkeypatch.setattr(core, "_list_all_files", fake_list)
+    monkeypatch.setattr(core, "_fetch_tags", fake_fetch)
+    return state
+
+
+# --- Marker-based skips ------------------------------------------------------
+
+
+def pytest_collection_modifyitems(config, items):
+    platform_skip = {
+        "macos": sys.platform != "darwin",
+        "windows": sys.platform != "win32",
+    }
+    for item in items:
+        for mark_name, should_skip in platform_skip.items():
+            if mark_name in item.keywords and should_skip:
+                item.add_marker(
+                    pytest.mark.skip(reason=f"requires {mark_name}")
+                )