Feature: Support subpath deployment for Go web UI

Add optional YAMA_WEB_BASE_PATH to mount the whole web UI (index, manifest,
static assets, /health, /api/devices, /ws) under a URL prefix such as
/remoter behind a reverse proxy. index.html now uses a __BASE_PATH__
placeholder substituted at serve time, the PWA manifest start_url is rooted
at the base path, and a mount wrapper redirects the bare prefix to prefix/
and strips it before routing. Include nginx, systemd, and env-file deployment
examples for the subpath setup.

The shared placeholder is also handled by the MFC server, which serves from
the domain root and strips __BASE_PATH__ (to "") so its web remote desktop
URLs stay rooted.

Co-Authored-By: deepseek-v4-pro

server/2015Remote/WebService.cpp

@@ -35,6 +35,16 @@ static std::string LoadBinaryResourceAsString(int resourceId) {
     return std::string(static_cast<const char*>(p), size);
 }
 
+// Replace all occurrences of `from` with `to` in-place.
+static void ReplaceAll(std::string& s, const std::string& from, const std::string& to) {
+    if (from.empty()) return;
+    size_t pos = 0;
+    while ((pos = s.find(from, pos)) != std::string::npos) {
+        s.replace(pos, from.length(), to);
+        pos += to.length();
+    }
+}
+
 // Challenge-response nonce storage (prevents replay attacks)
 static std::map<void*, std::string> s_ClientNonces;
 static std::mutex s_NonceMutex;
@@ -231,6 +241,10 @@ void CWebService::ServerThread(int port) {
 
     // Serve static HTML page and file downloads
     static std::string cachedHtml = GetWebPageHTML();
+    // server/web/index.html carries a __BASE_PATH__ placeholder for the Go
+    // server's subpath deployment. The MFC server serves from the domain root,
+    // so strip it (→ "") — otherwise the WebSocket/CSS/JS URLs break.
+    ReplaceAll(cachedHtml, "__BASE_PATH__", "");
     // Log loaded size; zero bytes means the RC BINARY resource is missing or
     // server/web/index.html was not embedded — check 2015Remote.rc and rebuild.
     Mprintf("[WebService] index.html loaded: %zu bytes\n", cachedHtml.size());