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/database.rs

@@ -1762,6 +1762,14 @@ impl Database {
         Ok(())
     }
 
+    pub fn set_verification_status(&self, id: i64, status: &str) -> SqlResult<()> {
+        self.conn.execute(
+            "UPDATE appimages SET verification_status = ?2 WHERE id = ?1",
+            params![id, status],
+        )?;
+        Ok(())
+    }
+
     // --- Launch statistics ---
 
     pub fn get_top_launched(&self, limit: i32) -> SqlResult<Vec<(String, u64)>> {