Make client API calls relative instead of hardcoded localhost:5144

The client always called a hardcoded http://localhost:5144, which only
worked when browser and server shared the same "localhost" — breaks for
any real remote deployment, since the browser would try to reach that
port on the visitor's own machine instead of the actual server.

Now every environment routes /api/* to the server under the same origin
the page was loaded from, so the client code needs no per-environment URL:
- Docker (client container's own nginx) proxies /api/ to the server
  container.
- `npm run dev` (Vite) proxies /api to localhost:5144 via server.proxy.
- Production nginx (reverse proxy + TLS) just needs to forward everything
  to the client container, which already knows how to route /api itself.

Also incidentally removes CORS from the picture everywhere, since none of
these setups make a cross-origin request anymore.

src/Client/Shared/JsonWire.fs

@@ -10,7 +10,13 @@ open Fable.Core
 open Fable.Core.JsInterop
 open Domain
 
-let private serverUrl = "http://localhost:5144"
+// Relative on purpose — the server is always reached through whatever's
+// serving this page under `/api/*`: the client's own nginx in Docker
+// (see Client/nginx.conf), Vite's dev-server proxy (vite.config.js) for
+// `npm run dev`, or the host reverse proxy in production. This also means
+// the browser never makes a cross-origin request, so CORS never enters
+// into it for any of these setups.
+let private serverUrl = ""
 
 [<Emit("Object.prototype.hasOwnProperty.call($0, $1)")>]
 let hasKey (_o: obj) (_key: string) : bool = jsNative

src/Client/nginx.conf

@@ -2,6 +2,17 @@ server {
     listen 80;
     root /usr/share/nginx/html;
 
+    # No trailing slash on proxy_pass here — the full matched URI (including
+    # the /api/ prefix) is forwarded unchanged, matching Giraffe's route
+    # definitions in Server/Routes.fs, which all start with /api/.
+    location /api/ {
+        proxy_pass http://server:8080;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+    }
+
     location / {
         try_files $uri $uri/ /index.html;
     }