feat: launch Chrome in app mode for chromeless window

python -m src.gui now opens Chrome with --app flag, hiding the address
bar, tabs, and bookmarks bar. Falls back to default browser if Chrome
is not found. Headless flag passed via CLI so streamlit run directly
still auto-opens normally.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/gui/__main__.py

@@ -1,8 +1,70 @@
-"""Allow running as ``python -m src.gui``."""
+"""Allow running as ``python -m src.gui``.
 
+Launches Streamlit with headless=true and opens Chrome in --app mode
+so the window has no address bar, tabs, or bookmarks — looks like a
+native desktop app.
+"""
+
+import os
+import shutil
 import subprocess
 import sys
+import threading
+import time
 from pathlib import Path
 
+_URL = "http://localhost:8501"
+
+
+def _find_chrome() -> str | None:
+    """Return the path to a Chrome/Chromium executable, or None."""
+    # Check PATH first
+    for name in ("chrome", "google-chrome", "google-chrome-stable", "chromium", "chromium-browser"):
+        found = shutil.which(name)
+        if found:
+            return found
+
+    # Windows common install locations
+    if sys.platform == "win32":
+        candidates = [
+            Path(os.environ.get("PROGRAMFILES", r"C:\Program Files"))
+            / "Google" / "Chrome" / "Application" / "chrome.exe",
+            Path(os.environ.get("PROGRAMFILES(X86)", r"C:\Program Files (x86)"))
+            / "Google" / "Chrome" / "Application" / "chrome.exe",
+            Path(os.environ.get("LOCALAPPDATA", ""))
+            / "Google" / "Chrome" / "Application" / "chrome.exe",
+        ]
+        for c in candidates:
+            if c.exists():
+                return str(c)
+
+    # macOS
+    if sys.platform == "darwin":
+        mac_chrome = Path("/Applications/Google Chrome.app/Contents/MacOS/Google Chrome")
+        if mac_chrome.exists():
+            return str(mac_chrome)
+
+    return None
+
+
+def _open_app_window() -> None:
+    """Wait for the server, then open Chrome in app mode (no address bar)."""
+    time.sleep(2)
+    chrome = _find_chrome()
+    if chrome:
+        subprocess.Popen([chrome, f"--app={_URL}"])
+    else:
+        # Fallback: open in default browser (will have address bar)
+        import webbrowser
+        webbrowser.open(_URL)
+
+
 app_path = Path(__file__).parent / "app.py"
-subprocess.run([sys.executable, "-m", "streamlit", "run", str(app_path)])
+
+# Open the browser in a background thread so it doesn't block Streamlit
+threading.Thread(target=_open_app_window, daemon=True).start()
+
+subprocess.run([
+    sys.executable, "-m", "streamlit", "run", str(app_path),
+    "--server.headless", "true",
+])