feat(linux): detect Linglong sandbox via LINGLONG_APPID env

[dengbo11]

When running inside a Linglong container the LINGLONG_APPID environment variable is set by the runtime, but was not checked by runningInSandbox(). Applications like LocalSend that pass a theme icon name to tray_manager were getting the icon name rewritten into a sandbox-internal absolute path, because the sandbox guard didn't recognise Linglong.

Adding the env-var check is consistent with the Flatpak/Snap pattern and requires no packages to guess the app's filesystem layout.

[dengbo11]

Hi maintainers,

I encountered an issue when using tray_manager in a Flutter application (LocalSend) running inside a Linglong (如意玲珑) sandbox on deepin 25. The tray icon showed a default
application-x-desktop icon instead of the app's actual icon.

After investigation, I found that runningInSandbox() does not recognize the Linglong environment, so the icon path is treated as a non-sandbox path and gets rewritten into a sandbox-
internal absolute path that the host's tray icon provider cannot read.

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What is Linglong?

Linglong (also known as Linyaps / 如意玲珑) is an open-source, cross-distribution Linux package format developed by the OpenAtom community.
Like Flatpak and Snap, it provides sandboxed application isolation via Linux kernel namespaces (mount, PID, network, etc.) and an OCI-compatible container runtime.

Key properties relevant to sandbox detection:

• The runtime always sets the LINGLONG_APPID environment variable inside the container, containing the application's reverse-domain ID (e.g., org.localsend). This is the canonical
  way to detect a Linglong sandbox.
• The application root inside the sandbox is /opt/apps/<APP_ID>/files/, which is not visible to host processes.
• The host's /run/user/<uid> is mounted into the container at the same path, so files written there are host-visible.
• Application icons from share/icons/hicolor/ are exported to the host at /var/lib/linglong/entries/share/icons/hicolor/, making them resolvable by QIcon::fromTheme().

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Why this change is needed

runningInSandbox() currently checks for FLATPAK_ID, SNAP, container, and /.dockerenv — but not LINGLONG_APPID. When a Flutter app inside Linglong calls
trayManager.setIcon(iconPath):

1. runningInSandbox() returns false → the sandbox guard is bypassed.
2. path.joinAll prepends the Flutter asset bundle path, turning the icon path into a sandbox-internal absolute path (e.g., /opt/apps/org.localsend/files/.../logo-32-white.png).
3. The host's tray icon provider receives this path via SNI, cannot access it, and falls back to the default icon.
4. No IconPixmap is provided by app_indicator_set_icon_full, so there is no fallback.

This is the same class of problem that Flatpak and Snap already solve — the sandbox guard simply doesn't know about Linglong. Adding LINGLONG_APPID to the detection list is the minimal,
consistent fix.

The LINGLONG_APPID environment variable is set unconditionally by the Linglong runtime (see [container_cfg_builder.cpp:1532](https://github.com/OpenAtom-Linyaps/linyaps/blob/main/libs/
oci-cfg-generators/src/linglong/oci-cfg-generators/container_cfg_builder.cpp#L1532)), so it is a reliable detection mechanism.

With this change, application code inside Linglong can pass a theme icon name (e.g., localsend_app) or a host-visible absolute path to trayManager.setIcon(), and the icon will be
correctly resolved by the host.

Best regards