fix: validate SSH key files have valid headers

Only include files in SSH key listing if they contain valid
private key headers (BEGIN ... PRIVATE KEY), regardless of
whether they have standard key filenames.

Co-authored-by: Ellie Huxtable <ellie@users.noreply.github.com>

Author: github-actions[bot]

backend/src/commands/ssh.rs

@@ -69,8 +69,8 @@ pub async fn list_ssh_keys() -> Result<Vec<SshKeyInfo>, String> {
         let is_standard = standard_names.contains(&name.as_str());
         let key_type = detect_key_type(&path);
 
-        // Only include if it looks like a private key
-        if key_type.is_some() || is_standard {
+        // Only include if it has valid private key headers
+        if key_type.is_some() {
             keys.push(SshKeyInfo {
                 name,
                 path: path.to_string_lossy().to_string(),