Add run.py launcher (IDLE/double-click/terminal entry point)

run.py

@@ -0,0 +1,58 @@
+"""
+Launcher for the Satellite Change Detection web app.
+
+Ways to run this:
+  - Python IDLE:  open this file, then Run > Run Module (F5)
+  - Double-click: works if .py files are associated with Python
+  - Terminal:     python run.py
+
+It starts the FastAPI server with uvicorn and opens your browser.
+Stop the server with Ctrl+C in the terminal (or close the IDLE shell window).
+"""
+import os
+import sys
+import threading
+import time
+import webbrowser
+
+HOST = "127.0.0.1"
+PORT = 8000
+
+
+def _open_browser_later(url: str):
+    # Give the server a moment to start, then open the browser once.
+    time.sleep(1.5)
+    try:
+        webbrowser.open(url)
+    except Exception:
+        pass
+
+
+def main():
+    # Make sure `import app.main` works no matter where this is launched from.
+    here = os.path.dirname(os.path.abspath(__file__))
+    os.chdir(here)
+    if here not in sys.path:
+        sys.path.insert(0, here)
+
+    try:
+        import uvicorn
+    except ImportError:
+        print("ERROR: dependencies are not installed.")
+        print("Run this first:  pip install -r requirements.txt")
+        sys.exit(1)
+
+    url = f"http://{HOST}:{PORT}"
+    print("Starting Satellite Change Detection...")
+    print(f"Open in your browser:  {url}")
+    print("Press Ctrl+C to stop.\n")
+
+    threading.Thread(target=_open_browser_later, args=(url,), daemon=True).start()
+
+    # reload=False keeps it simple and IDLE-friendly. For live-reload during
+    # development, run instead:  uvicorn app.main:app --reload --port 8000
+    uvicorn.run("app.main:app", host=HOST, port=PORT, reload=False)
+
+
+if __name__ == "__main__":
+    main()