v0.27.0: Cassette invites — one paste connects two friends

- Connections ▸ Share Library with a Friend… makes a one-time invite code
  (device id, display name, token) and the inviter's send-only folder.
- Connections ▸ Add Friend Library… takes the pasted code: adds the
  inviter, shares our folder, and accepts theirs receive-only when it's
  offered back.
- Syncthing hides an unknown device's folders (verified on 1.30), so the
  inviter probes a new knock only while an invite is open: added with
  nothing shared, completed on the right token, otherwise removed and
  never probed again. Used and cancelled codes go nowhere.
- Sync Settings lists open invites (Cancel Invite) and gives each friend a
  page: avatar, plain-language status with a next step, last seen, last
  synced, Remove Friend. Removals are staged until OK.
- scripts/cassette_pair.py runs throwaway Syncthings on loopback;
  pytest --syncthing drives a real three-machine handshake.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/conftest.py

@@ -79,3 +79,51 @@ def jpeg_bytes(tmp_path):
     if result.returncode != 0:
         pytest.skip("ffmpeg could not generate jpeg fixture")
     return path.read_bytes()
+
+
+# ---- real-Syncthing tests (Cassette) ----
+# Two throwaway Syncthing instances on loopback (scripts/cassette_pair.py).
+# Slow-ish (a handshake takes Syncthing some seconds), so opt-in:
+#     python3 -m pytest -m syncthing --syncthing
+
+def pytest_addoption(parser):
+    parser.addoption("--syncthing", action="store_true",
+                     help="run tests that start two real Syncthing instances")
+
+
+def pytest_configure(config):
+    config.addinivalue_line(
+        "markers", "syncthing: starts two real Syncthing instances "
+                   "(opt in with --syncthing)")
+
+
+def pytest_collection_modifyitems(config, items):
+    if config.getoption("--syncthing"):
+        return
+    skip = pytest.mark.skip(reason="needs --syncthing")
+    for item in items:
+        if "syncthing" in item.keywords:
+            item.add_marker(skip)
+
+
+@pytest.fixture
+def syncthing_pair(tmp_path):
+    """[alice, bob, carol] Instances from scripts/cassette_pair.py, stopped
+    after."""
+    if shutil.which("syncthing") is None:
+        pytest.skip("syncthing isn't installed")
+    import importlib.util
+    from pathlib import Path
+    spec = importlib.util.spec_from_file_location(
+        "cassette_pair",
+        Path(__file__).resolve().parent.parent / "scripts" / "cassette_pair.py")
+    pair = importlib.util.module_from_spec(spec)
+    import sys
+    sys.modules[spec.name] = pair  # dataclasses look their module up there
+    spec.loader.exec_module(pair)
+    out = tmp_path / "pair"
+    instances = pair.start(out, names=("alice", "bob", "carol"))
+    try:
+        yield instances
+    finally:
+        pair.stop(out)