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/tests/integration.rs

@@ -59,6 +59,7 @@ fn full_vault_workflow() {
         password: "supersecret123!".to_string(),
         notes: Some("my main account".to_string()),
         totp_secret: None,
+        group: None,
         created_at: "2024-01-01T00:00:00Z".to_string(),
         updated_at: "2024-01-01T00:00:00Z".to_string(),
     };
@@ -102,6 +103,7 @@ fn full_vault_workflow() {
             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(),
         },
     );