feat: customize login and lobby spaces from the panel

plugins/shared/src/main/java/best/lolicon/felis/link/ExperienceConfig.java

@@ -0,0 +1,43 @@
+package best.lolicon.felis.link;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Map;
+
+/** Player-facing settings on the persistent world volume, edited by the panel. */
+public final class ExperienceConfig {
+    public static final Path PATH = Path.of("felis-experience.json");
+    private final Map<?, ?> values;
+
+    public ExperienceConfig(Map<?, ?> values) {
+        this.values = values;
+    }
+
+    public static ExperienceConfig load(Path path) throws IOException {
+        if (!Files.exists(path)) {
+            return new ExperienceConfig(Map.of());
+        }
+        try {
+            Object value = Json.parse(Files.readString(path));
+            if (value instanceof Map<?, ?> map) {
+                return new ExperienceConfig(map);
+            }
+            throw new IllegalArgumentException("expected an object");
+        } catch (IllegalArgumentException e) {
+            throw new IOException("invalid " + path + ": " + e.getMessage(), e);
+        }
+    }
+
+    public String text(String key, String fallback) {
+        return values.get(key) instanceof String value ? value : fallback;
+    }
+
+    public boolean flag(String key, boolean fallback) {
+        return values.get(key) instanceof Boolean value ? value : fallback;
+    }
+
+    public long number(String key, long fallback) {
+        return values.get(key) instanceof Number value ? value.longValue() : fallback;
+    }
+}