feat(v1): settings camera connection test and preview

Settings tab gains a camera group (host/port/channel/user/masked password),
a background connection-test worker that probes one frame off the GUI
thread and shows it in a preview, and a save button that writes the
structured source to the untracked config.local.json. 60 tests pass;
Qt shell compiles and needs a Windows smoke.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

v1/tests/test_config.py

@@ -107,6 +107,28 @@ def test_structured_credentials_are_excluded_from_config_version(tmp_path):
     assert first.runtime_config_version == second.runtime_config_version
 
 
+def test_write_local_camera_source_round_trips_through_load_config(tmp_path):
+    from v1.config import write_local_camera_source
+
+    config_file = tmp_path / "config.local.json"
+    _write_config(config_file, {"id": "seed", "rtsp_url_env": "SILVER_POSE_RTSP_URL"})
+
+    write_local_camera_source(
+        config_file,
+        source_id="hik",
+        host="192.0.2.10",
+        port=554,
+        channel="102",
+        username="admin",
+        password="p@ss/w:d",
+    )
+    config = load_config(config_file)
+
+    assert config.source_url == "rtsp://admin:p%40ss%2Fw%3Ad@192.0.2.10:554/Streaming/Channels/102"
+    assert config.source_mode == "stream"
+    assert config.source_id == "hik"
+
+
 def test_public_example_config_has_no_embedded_credentials():
     from pathlib import Path