Move socket into separate directory (#8030)

This is mostly to ease setup and configuration with sandboxed browsers.

The socket currently existing in `$XDG_RUNTIME_DIR`. When sandboxing a browser, it would be unsafe to mount this directory inside the sandbox.
Mounting the socket into the sandbox's filesystem is also not possible in cases where KeePassXC is [re]started after the browser has started.

This commit moves the socket into its own isolated subdirectory, which can be safely mounted into sandboxes. Sandbox engines can create the directory themselves (in case the browser starts before KeePassXC). Both Flatpak and Firejail support this configuration.

A symlink is also created, linking the previous location to the new location. This is meant for backwards compatibility and should eventually be dropped.

The directory can't be named `org.keepassxc.KeePassXC.BrowserServer`,
since that would collide with the symlink. Instead, the directory has been created to match the format used for Flatpak builds, which make it a bit less of a snowflake build, while following accepted conventions.

Given that the preferred path now matches what Flatpak uses, the block handling Flatpak and non-Flatpak is now the same.

If `$XDG_RUNTIME_DIR` is undefined, the temporary directory is used, though reading the socket from this location is discouraged.

Closes: #8018
References: #6741

src/browser/BrowserShared.cpp

@@ -19,6 +19,7 @@
 
 #include "config-keepassx.h"
 
+#include <QDir>
 #include <QStandardPaths>
 #if defined(KEEPASSXC_DIST_SNAP)
 #include <QProcessEnvironment>
@@ -31,14 +32,22 @@ namespace BrowserShared
         const auto serverName = QStringLiteral("/org.keepassxc.KeePassXC.BrowserServer");
 #if defined(KEEPASSXC_DIST_SNAP)
         return QProcessEnvironment::systemEnvironment().value("SNAP_USER_COMMON") + serverName;
-#elif defined(KEEPASSXC_DIST_FLATPAK)
-        return QStandardPaths::writableLocation(QStandardPaths::RuntimeLocation) + "/app/" + "org.keepassxc.KeePassXC"
-               + serverName;
 #elif defined(Q_OS_UNIX) && !defined(Q_OS_MACOS)
-        // Use XDG_RUNTIME_DIR instead of /tmp if it's available
+        // This returns XDG_RUNTIME_DIR or else a temporary subdirectory.
         QString path = QStandardPaths::writableLocation(QStandardPaths::RuntimeLocation);
-        return path.isEmpty() ? QStandardPaths::writableLocation(QStandardPaths::TempLocation) + serverName
-                              : path + serverName;
+
+        // Put the socket in a dedicated directory.
+        // This directory will be easily mountable by sandbox containers.
+        QString subPath = path + "/app/org.keepassxc.KeePassXC/";
+        QDir().mkpath(subPath);
+
+        QString socketPath = subPath + serverName;
+#ifndef KEEPASSXC_DIST_FLATPAK
+        // Create a symlink at the legacy location for backwards compatibility.
+        QFile::link(socketPath, path + serverName);
+#endif
+
+        return socketPath;
 #elif defined(Q_OS_WIN)
         // Windows uses named pipes
         return serverName + "_" + qgetenv("USERNAME");