fix(settings): don't let a stale DLC path block saving the Demucs server address (#795)

The v3 Settings "Save" button posts dlc_dir together with demucs_server_url,
default_arrangement and av_offset_ms in one request. POST /api/settings
validated dlc_dir first and early-returned "DLC directory not found" before it
ever processed demucs_server_url, so on a machine whose DLC path doesn't resolve
(fresh install, unplugged/network drive, a path carried over from another
machine) setting the Demucs server address silently failed — reported in
got-feedBack/feedBack-demucs-server#3 (macOS 07-05 nightly).

- server: a non-resolving dlc_dir is now recorded as a warning and skipped
  rather than aborting the whole POST, so the co-submitted keys still persist.
  The bad path is surfaced via a new additive `warnings` field and folded into
  `message` so the settings status line still shows it.
- client (v3): the Demucs input now autosaves on blur/enter via a single-key
  persistSetting POST, like every other v3 setting, so it never depends on the
  coupled Save button.
- tests: cover the decoupling and the unchanged happy path.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_settings_api.py

@@ -219,6 +219,47 @@ def test_dlc_dir_empty_string_clears(client, tmp_path):
     assert _read_cfg(tmp_path)["dlc_dir"] == ""
 
 
+def test_unresolvable_dlc_dir_does_not_block_other_keys(client, tmp_path):
+    # Regression (feedBack-demucs-server#3): the v3 "Save" button next to the
+    # Demucs field bundles dlc_dir with demucs_server_url in one POST. A DLC
+    # path that doesn't resolve on THIS machine (stale value / unplugged drive)
+    # must not abort the whole request — the co-submitted demucs_server_url has
+    # to persist, and the bad path is surfaced as a warning rather than a hard
+    # error that drops every other key.
+    missing = str(tmp_path / "does-not-exist")
+    r = client.post("/api/settings", json={
+        "dlc_dir": missing,
+        "demucs_server_url": "http://demucs.example:7865",
+        "default_arrangement": "Lead",
+    })
+    assert r.status_code == 200
+    body = r.json()
+    # No hard error; the bad path is reported as a warning.
+    assert "error" not in body
+    assert any("does-not-exist" in w for w in body.get("warnings", []))
+    assert "does-not-exist" in body["message"]
+    cfg = _read_cfg(tmp_path)
+    # The valid keys persisted...
+    assert cfg["demucs_server_url"] == "http://demucs.example:7865"
+    assert cfg["default_arrangement"] == "Lead"
+    # ...and the unresolvable path was NOT written.
+    assert cfg.get("dlc_dir", "") != missing
+
+
+def test_valid_dlc_dir_still_reports_song_count(client, tmp_path):
+    # The happy path is unchanged: a resolvable DLC dir persists and the
+    # response message still carries the "N song files found" summary (no
+    # warnings key when nothing went wrong).
+    dlc = tmp_path / "dlc"
+    dlc.mkdir()
+    r = client.post("/api/settings", json={"dlc_dir": str(dlc)})
+    assert r.status_code == 200
+    body = r.json()
+    assert "warnings" not in body
+    assert "song files found" in body["message"]
+    assert _read_cfg(tmp_path)["dlc_dir"] == str(dlc)
+
+
 @pytest.mark.parametrize("key", ["default_arrangement", "demucs_server_url"])
 def test_string_key_null_is_noop(client, tmp_path, key):
     # Match the dlc_dir contract: null preserves the on-disk value.