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.ts

@@ -0,0 +1,44 @@
+import { normalizeLanguageCode } from "@/lib/languageNames";
+
+/** Valid private-use BCP 47 tag understood by the server as item-original. */
+export const ORIGINAL_METADATA_LANGUAGE = "x-silo-original";
+
+export type MetadataLanguageOverrides = Record<string, string>;
+
+/**
+ * Treat the settings response as untrusted JSON at the component boundary.
+ * The server validates writes, but this also keeps a stale/broken cached value
+ * from making the editor unusable.
+ */
+export function normalizeMetadataLanguageOverrides(value: unknown): MetadataLanguageOverrides {
+  if (value === null || typeof value !== "object" || Array.isArray(value)) return {};
+
+  const normalized: MetadataLanguageOverrides = {};
+  for (const [source, target] of Object.entries(value)) {
+    const sourceCode = normalizeLanguageCode(source);
+    if (!/^[a-z]{2,3}$/.test(sourceCode) || typeof target !== "string" || !target.trim()) {
+      continue;
+    }
+    normalized[sourceCode] = target.trim();
+  }
+  return normalized;
+}
+
+export function withMetadataLanguageOverride(
+  overrides: MetadataLanguageOverrides,
+  source: string,
+  target: string,
+): MetadataLanguageOverrides {
+  const sourceCode = normalizeLanguageCode(source);
+  if (!/^[a-z]{2,3}$/.test(sourceCode) || !target.trim()) return overrides;
+  return { ...overrides, [sourceCode]: target.trim() };
+}
+
+export function withoutMetadataLanguageOverride(
+  overrides: MetadataLanguageOverrides,
+  source: string,
+): MetadataLanguageOverrides {
+  const next = { ...overrides };
+  delete next[normalizeLanguageCode(source)];
+  return next;
+}