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-cli/src/main.rs

@@ -457,6 +457,7 @@ fn cmd_add() -> Result<()> {
         password,
         notes,
         totp_secret,
+        group: None,
         created_at: now.clone(),
         updated_at: now.clone(),
     };
@@ -476,6 +477,7 @@ fn cmd_add() -> Result<()> {
             name: name.clone(),
             url,
             username,
+            group: None,
             updated_at: now,
         },
     );
@@ -660,6 +662,7 @@ fn cmd_edit(query: String) -> Result<()> {
         password,
         notes,
         totp_secret,
+        group: entry.group,
         created_at: entry.created_at,
         updated_at: now.clone(),
     };
@@ -678,6 +681,7 @@ fn cmd_edit(query: String) -> Result<()> {
             name: name.clone(),
             url,
             username,
+            group: None,
             updated_at: now,
         },
     );