fix: Steam Deck/Ubuntu crash with jemalloc allocator

Root cause: glibc 2.41 has stricter heap validation that catches a
pre-existing race condition triggered by binary patching.

Changes:
- Add jemalloc auto-detection and usage on Linux
- Add auto-install via pkexec (graphical sudo prompt)
- Clean up clientPatcher.js (remove debug env vars)
- Add null-padding fix for shorter domain replacements
- Document investigation and solution

The launcher now:
1. Auto-detects jemalloc if installed
2. Offers to auto-install if missing (password prompt)
3. Falls back to MALLOC_CHECK_=0 if jemalloc unavailable

Install manually: sudo pacman -S jemalloc (Arch/Steam Deck)
                  sudo apt install libjemalloc2 (Debian/Ubuntu)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/managers/gameLauncher.js

@@ -24,6 +24,57 @@ try {
 
 const execAsync = promisify(exec);
 
+/**
+ * Try to auto-install jemalloc on Linux using pkexec (graphical sudo)
+ * Returns true if installation was successful
+ */
+async function tryInstallJemalloc() {
+  console.log('Linux: Attempting to auto-install jemalloc...');
+
+  // Detect package manager and get install command
+  let installCmd = null;
+  try {
+    await execAsync('which pacman');
+    installCmd = 'pacman -S --noconfirm jemalloc';
+  } catch (e) {
+    try {
+      await execAsync('which apt');
+      installCmd = 'apt install -y libjemalloc2';
+    } catch (e2) {
+      try {
+        await execAsync('which dnf');
+        installCmd = 'dnf install -y jemalloc';
+      } catch (e3) {
+        console.log('Linux: Could not detect package manager for auto-install');
+        return false;
+      }
+    }
+  }
+
+  // Try pkexec first (graphical sudo), fall back to sudo
+  const sudoCommands = ['pkexec', 'sudo'];
+  for (const sudoCmd of sudoCommands) {
+    try {
+      await execAsync(`which ${sudoCmd}`);
+      console.log(`Linux: Installing jemalloc with: ${sudoCmd} ${installCmd}`);
+      await execAsync(`${sudoCmd} ${installCmd}`, { timeout: 120000 });
+      console.log('Linux: jemalloc installed successfully');
+      return true;
+    } catch (e) {
+      if (e.killed) {
+        console.log('Linux: Install timed out');
+      } else if (e.code === 126 || e.code === 127) {
+        continue;
+      } else {
+        console.log(`Linux: Install failed with ${sudoCmd}: ${e.message}`);
+      }
+    }
+  }
+
+  console.log('Linux: Auto-install failed, manual installation required');
+  return false;
+}
+
 // Fetch tokens from the auth server (properly signed with server's Ed25519 key)
 async function fetchAuthTokens(uuid, name) {
   const authServerUrl = getAuthServerUrl();
@@ -285,6 +336,59 @@ exec "$REAL_JAVA" "\${ARGS[@]}"
    const gpuEnv = setupGpuEnvironment(gpuPreference);
    Object.assign(env, gpuEnv);
 
+   // Linux: Use jemalloc to fix "free(): invalid pointer" crash on glibc 2.41+ (Steam Deck, Ubuntu LTS)
+   // Root cause: glibc 2.41 has stricter heap validation that catches a pre-existing race condition
+   if (process.platform === 'linux') {
+     if (process.env.HYTALE_NO_JEMALLOC !== '1') {
+       const jemallocPaths = [
+         '/usr/lib/libjemalloc.so.2',                    // Arch Linux, Steam Deck
+         '/usr/lib/x86_64-linux-gnu/libjemalloc.so.2',   // Debian/Ubuntu
+         '/usr/lib64/libjemalloc.so.2',                  // Fedora/RHEL
+         '/usr/lib/libjemalloc.so',                      // Generic fallback
+         '/usr/lib/x86_64-linux-gnu/libjemalloc.so',     // Debian/Ubuntu fallback
+         '/usr/lib64/libjemalloc.so'                     // Fedora/RHEL fallback
+       ];
+
+       let jemalloc = null;
+       for (const p of jemallocPaths) {
+         if (fs.existsSync(p)) {
+           jemalloc = p;
+           break;
+         }
+       }
+
+       if (jemalloc) {
+         env.LD_PRELOAD = jemalloc + (env.LD_PRELOAD ? ':' + env.LD_PRELOAD : '');
+         console.log(`Linux: Using jemalloc allocator for stability (${jemalloc})`);
+       } else {
+         // Try auto-install
+         if (process.env.HYTALE_AUTO_INSTALL_JEMALLOC !== '0') {
+           const installed = await tryInstallJemalloc();
+           if (installed) {
+             for (const p of jemallocPaths) {
+               if (fs.existsSync(p)) {
+                 jemalloc = p;
+                 break;
+               }
+             }
+             if (jemalloc) {
+               env.LD_PRELOAD = jemalloc + (env.LD_PRELOAD ? ':' + env.LD_PRELOAD : '');
+               console.log(`Linux: Using jemalloc after auto-install (${jemalloc})`);
+             }
+           }
+         }
+
+         if (!jemalloc) {
+           env.MALLOC_CHECK_ = '0';
+           console.log('Linux: jemalloc not found - install with: sudo pacman -S jemalloc (Arch) or sudo apt install libjemalloc2 (Debian/Ubuntu)');
+           console.log('Linux: Using fallback MALLOC_CHECK_=0 (may still crash on glibc 2.41+)');
+         }
+       }
+     } else {
+       console.log('Linux: jemalloc disabled by HYTALE_NO_JEMALLOC=1');
+     }
+   }
+
   try {
     let spawnOptions = {
       stdio: ['ignore', 'pipe', 'pipe'],