refactor: simplify _get_ws_url_from_request

Consolidate nested helper functions and reduce complexity:
- Inline header extraction with first_header helper
- Use rpartition for cleaner host:port splitting
- Simplify control flow with loop over header candidates

Addresses REFACTORING.md item about simplifying _get_ws_url_from_request.

src/textual_webterm/local_server.py

@@ -579,39 +579,38 @@ class LocalServer:
 
     def _get_ws_url_from_request(self, request: web.Request, route_key: str) -> str:
         """Build WebSocket URL honoring reverse proxies and port mapping."""
+        # Extract forwarded headers (take first value if comma-separated)
+        def first_header(name: str) -> str:
+            return request.headers.get(name, "").split(",")[0].strip().lower()
 
-        forwarded_proto = request.headers.get("X-Forwarded-Proto", "").split(",")[0].strip().lower()
-        forwarded_host = request.headers.get("X-Forwarded-Host", "").split(",")[0].strip()
-        forwarded_port = request.headers.get("X-Forwarded-Port", "").split(",")[0].strip()
+        forwarded_proto = first_header("X-Forwarded-Proto")
+        forwarded_host = first_header("X-Forwarded-Host")
+        forwarded_port = first_header("X-Forwarded-Port")
 
-        def _pick_proto() -> str:
-            if forwarded_proto in ("https", "wss"):
-                return "wss"
-            if forwarded_proto in ("http", "ws"):
-                return "ws"
-            return "wss" if request.secure else "ws"
+        # Determine WebSocket protocol
+        if forwarded_proto in ("https", "wss"):
+            ws_proto = "wss"
+        elif forwarded_proto in ("http", "ws"):
+            ws_proto = "ws"
+        else:
+            ws_proto = "wss" if request.secure else "ws"
 
-        def _split_host_port(host: str) -> tuple[str, str]:
-            if not host:
-                return "", ""
-            if ":" in host:
-                return host.rsplit(":", 1)
-            return host, ""
-
-        ws_proto = _pick_proto()
-        ws_host, ws_port = _split_host_port(forwarded_host)
-
-        if not ws_host:
-            host_header = request.headers.get("Host", "")
-            ws_host, ws_port = _split_host_port(host_header)
+        # Determine host and port (priority: forwarded > Host header > server config)
+        ws_host, ws_port = "", ""
+        for candidate in (forwarded_host, request.headers.get("Host", "")):
+            if candidate:
+                ws_host, _, ws_port = candidate.rpartition(":")
+                if not ws_host:  # No colon found, entire string is host
+                    ws_host, ws_port = candidate, ""
+                break
 
         if not ws_host:
             ws_host = "localhost" if self.host == "0.0.0.0" else self.host
             ws_port = str(self.port)
 
-        if not ws_port and forwarded_port:
-            ws_port = forwarded_port
+        ws_port = ws_port or forwarded_port
 
+        # Include port in URL only for non-standard ports
         if ws_port and ws_port not in ("80", "443"):
             return f"{ws_proto}://{ws_host}:{ws_port}/ws/{route_key}"
         if not ws_port and self.port not in (80, 443):