fix: handle directory access for static web-ui routes

When accessing /static/web-ui without trailing slash, the request would
fail with "RuntimeError: File at path ... is not a file" because:

1. The route /static/web-ui/{file_path:path} doesn't match paths without
   trailing slash (Starlette's path regex requires the /)
2. The request falls through to StaticFiles mount, which tries to serve
   the directory as a file

This fix:
- Sets html=True on StaticFiles mount to automatically redirect directory
  URLs to trailing slash versions
- Adds os.path.isdir() check to handle empty file_path gracefully

Fixes #2680

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

gns3server/api/routes/index.py

@@ -52,7 +52,7 @@ async def web_ui(file_path: str):
 
     static = get_resource(file_path)
 
-    if static is None or not os.path.exists(static):
+    if static is None or not os.path.exists(static) or os.path.isdir(static):
         static = get_resource(os.path.join("static", "web-ui", "index.html"))
 
     if static is None:

gns3server/api/server.py

@@ -73,7 +73,7 @@ def get_application() -> FastAPI:
 
     application.include_router(index.router, tags=["Index"])
     application.include_router(controller.router, prefix="/v3")
-    application.mount("/static", StaticFiles(packages=[('gns3server', 'static')]), name="static")
+    application.mount("/static", StaticFiles(packages=[('gns3server', 'static')], html=True), name="static")
     application.mount("/v3/compute", compute_api, name="compute")
 
     return application