fix: ensure any non-existent file in the server is routed to the corresponding SPA

try_files avoids 404s, redirecting to SPA root; catch-all handles unknown paths for selfhost-web.

aio.Caddyfile

@@ -1,13 +1,26 @@
 :3000 {
-  root * /site/selfhost-web
-  file_server
-
-  handle_path /admin* {
-    root * /site/sh-admin
+    # Serve the `selfhost-web` SPA by default
+    root * /site/selfhost-web
     file_server
-  }
 
-  handle_path /backend* {
-    reverse_proxy localhost:3170
-  }
+    # Handle requests under `/admin*`
+    handle_path /admin* {
+        root * /site/sh-admin
+        file_server
+
+        # Ensures any non-existent file in the server is routed to the SPA
+        try_files {path} /
+    }
+
+    # Handle requests under `/backend*` path
+    handle_path /backend* {
+        reverse_proxy localhost:3170
+    }
+
+    # Catch-all route for unknown paths, serves `selfhost-web` SPA
+    handle {
+        root * /site/selfhost-web
+        file_server
+        try_files {path} /
+    }
 }