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/config.py

@@ -114,6 +114,38 @@ def _resolve_path(config_path: Path, value: Any, field_name: str) -> Path:
     return (config_path.parent / raw_path).resolve()
 
 
+def write_local_camera_source(
+    config_path: Path,
+    source_id: str,
+    host: str,
+    port: int,
+    channel: str,
+    username: str,
+    password: str,
+    mode: str = "stream",
+) -> Path:
+    """Persist a structured camera source into an untracked local config file.
+
+    Only ever call this on ``config.local.json`` (git-ignored). Credentials are
+    written in plaintext by design of the chosen policy; the public example must
+    never receive them.
+    """
+
+    path = Path(config_path)
+    data = json.loads(path.read_text(encoding="utf-8")) if path.exists() else {}
+    data["source"] = {
+        "id": str(source_id),
+        "host": str(host),
+        "port": int(port),
+        "channel": str(channel),
+        "username": str(username),
+        "password": str(password),
+        "mode": mode,
+    }
+    path.write_text(json.dumps(data, ensure_ascii=False, indent=2) + "\n", encoding="utf-8")
+    return path
+
+
 def load_config(path: Path) -> AppConfig:
     """Load one public/local configuration pair without persisting credentials.