feat: add blacked visual theme with full-screen dark overlay effect

Implements a complete blacked-out visual state where all content is replaced by a solid black overlay covering the entire viewport. The overlay includes smooth fade-in animation and prevents any user interaction with underlying elements. This creates an immersive dark experience suitable for cinematic transitions or focus modes.

src/rupload/app.py

@@ -278,7 +278,7 @@ async def handle_index(request: web.Request):
     if "is available here: " in message:
         url = message[message.find("here: ") + len("here: ") :]
         message = message[: message.find("here: ") + len("here: ")]
-        message += f"<a href=\"{url}\">"
+        message += f'<a href="{url}">'
         message += f"{url}</a>"
     if message:
         message += "<br /><br />"