feat: add group field to Entry and ManifestEntry

Add optional group: Option<String> to both Entry and ManifestEntry for
logical organization (e.g. "work", "personal"). Backwards-compatible via
skip_serializing_if so existing vaults deserialize with group: None.
Includes three new tests verifying round-trip and legacy deserialization.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/idfoto-core/src/vault.rs

@@ -94,6 +94,7 @@ mod tests {
             password: "secret123".to_string(),
             notes: None,
             totp_secret: None,
+            group: None,
             created_at: "2024-01-01T00:00:00Z".to_string(),
             updated_at: "2024-01-01T00:00:00Z".to_string(),
         }
@@ -122,6 +123,7 @@ mod tests {
                 name: "GitHub".to_string(),
                 url: Some("https://github.com".to_string()),
                 username: Some("alice".to_string()),
+                group: None,
                 updated_at: "2024-01-01T00:00:00Z".to_string(),
             },
         );