docs(advanced): add app self-check documentation

- describe the application self-check feature
- explain how it works and how to enable it
- list build requirements and platform compatibility
- include a security note about the feature

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+---
+title: "Application Self-Check"
+sidebar:
+  label: Application Self-Check
+---
+
+When Self-Check is enabled, GpgFrontend validates the digital signatures of its
+core dynamic libraries and components at startup. This ensures that the
+application’s essential code has not been altered, replaced, or
+corrupted—whether accidentally or through malicious interference.
+
+:::tip[Note]
+
+The Application Self-Check feature is available starting from version
+2.1.9. In earlier versions, this integrity verification mechanism is not
+present.
+
+:::
+
+## How It Works
+
+- During the build process, a cryptographic key pair is generated automatically
+  for each build.
+- The application libraries (DLLs) are signed using the private key, and only
+  the corresponding public key is embedded within the application resources.
+- After signing, the private key is discarded, making it impossible to forge new
+  signatures for tampered binaries.
+- At runtime, GpgFrontend verifies each critical library against its signature
+  using the embedded public key. If any library fails validation, a warning is
+  displayed and the issue is logged.
+
+## Enabling Self-Check
+
+Self-Check can be enabled by adding the following line to your ENV.ini
+configuration file in the application’s working directory:
+
+```ini
+SelfCheck=true
+```
+
+With this setting active, GpgFrontend will automatically perform a binary
+integrity check at startup.
+
+## Build Requirements
+
+- The self-check signing process requires the `GPGFRONTEND_SIGN_BUILT_BINARY`
+  parameter to be set during the build. This triggers the generation of the
+  signing key pair and the signing of all relevant binaries.
+- The signing key pair is unique to each build. The private key is securely
+  deleted after signing, ensuring that no one—including the developers—can
+  generate valid signatures for modified or malicious binaries.
+
+## Platform Compatibility
+
+- Windows: The self-check feature is currently effective only on Windows
+  platforms. On Windows, dynamic library files are stable after build and
+  signing, allowing signature verification to work reliably.
+- macOS and Linux: Due to common post-build modifications such as codesign,
+  rpath adjustment, or packaging processes (e.g., AppImage, Flatpak), dynamic
+  library files on macOS and Linux often change after build. As a result,
+  digital signatures become invalid, and self-check is not currently effective
+  on these platforms.
+
+## Security Note
+
+The Application Self-Check adds an important layer of security by helping to
+ensure that GpgFrontend’s executable code remains authentic and untampered. It
+is particularly recommended for security-sensitive deployments or when running
+GpgFrontend on shared or potentially untrusted systems.