Add download verification with signature and SHA256 support

New verification module with GPG signature checking and SHA256 hash
computation. Security tab shows verification status, embedded signature
check button, and manual SHA256 input for verifying downloads.

src/core/mod.rs

@@ -14,5 +14,6 @@ pub mod orphan;
 pub mod report;
 pub mod security;
 pub mod updater;
+pub mod verification;
 pub mod watcher;
 pub mod wayland;