fix(core): SymbolCharset needs content="value" for Custom(String)

Same latent bug as TrashRetention/HistoryRetention — serde's
internally-tagged repr cannot merge a newtype primitive payload
into a tag object. Add regression test for Custom round-trip.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/idfoto-core/src/settings.rs

@@ -95,7 +95,7 @@ pub struct CharClasses {
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
-#[serde(tag = "kind", rename_all = "snake_case")]
+#[serde(tag = "kind", content = "value", rename_all = "snake_case")]
 pub enum SymbolCharset {
     SafeOnly,
     Extended,
@@ -170,4 +170,15 @@ mod tests {
             _ => panic!("expected Random default"),
         }
     }
+
+    #[test]
+    fn symbol_charset_custom_round_trips() {
+        let c = SymbolCharset::Custom("!@#".into());
+        let json = serde_json::to_string(&c).unwrap();
+        let parsed: SymbolCharset = serde_json::from_str(&json).unwrap();
+        match parsed {
+            SymbolCharset::Custom(s) => assert_eq!(s, "!@#"),
+            other => panic!("expected Custom, got {:?}", other),
+        }
+    }
 }