feat(metadata): add original-language preferences (#526)

* feat(metadata): add original-language preferences

* fix(settings): show metadata exceptions immediately

* docs(settings): add metadata language screenshot

* fix(settings): make language exceptions responsive

* fix(settings): standardize language display names

web/src/lib/metadataLanguagePreferences.test.ts

@@ -0,0 +1,30 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  ORIGINAL_METADATA_LANGUAGE,
+  normalizeMetadataLanguageOverrides,
+  withMetadataLanguageOverride,
+  withoutMetadataLanguageOverride,
+} from "./metadataLanguagePreferences";
+
+describe("metadata language preferences", () => {
+  it("normalizes source aliases and drops malformed cached entries", () => {
+    expect(
+      normalizeMetadataLanguageOverrides({
+        nor: ORIGINAL_METADATA_LANGUAGE,
+        JA: "en",
+        invalid_language: "fr",
+        de: 42,
+      }),
+    ).toEqual({ no: ORIGINAL_METADATA_LANGUAGE, ja: "en" });
+  });
+
+  it("adds and removes one original-language exception without mutating the input", () => {
+    const initial = { ja: "en" };
+    const added = withMetadataLanguageOverride(initial, "nor", ORIGINAL_METADATA_LANGUAGE);
+
+    expect(added).toEqual({ ja: "en", no: ORIGINAL_METADATA_LANGUAGE });
+    expect(initial).toEqual({ ja: "en" });
+    expect(withoutMetadataLanguageOverride(added, "no")).toEqual({ ja: "en" });
+  });
+});