feat(i18n): add German translations and key parity test

- Create all 6 German namespace JSON files (common, collection, threads, setups, onboarding, settings)
- Register German locale in i18n configuration with supportedLngs
- Add key parity test ensuring en/de have identical key structures
- All 19 locale parity tests pass, all 15 formatter tests pass

Phase 34, Plan 05

tests/i18n/locales.test.ts

@@ -0,0 +1,76 @@
+import { readdirSync, readFileSync } from "node:fs";
+import { join } from "node:path";
+import { describe, expect, test } from "bun:test";
+
+const LOCALES_DIR = join(import.meta.dir, "../../src/client/locales");
+
+function flattenKeys(
+	obj: Record<string, unknown>,
+	prefix = "",
+): string[] {
+	const keys: string[] = [];
+	for (const [key, value] of Object.entries(obj)) {
+		const fullKey = prefix ? `${prefix}.${key}` : key;
+		if (typeof value === "object" && value !== null && !Array.isArray(value)) {
+			keys.push(
+				...flattenKeys(value as Record<string, unknown>, fullKey),
+			);
+		} else {
+			keys.push(fullKey);
+		}
+	}
+	return keys.sort();
+}
+
+function loadLocale(
+	locale: string,
+): Record<string, Record<string, unknown>> {
+	const dir = join(LOCALES_DIR, locale);
+	const files = readdirSync(dir).filter((f) => f.endsWith(".json"));
+	const result: Record<string, Record<string, unknown>> = {};
+	for (const file of files) {
+		const ns = file.replace(".json", "");
+		result[ns] = JSON.parse(readFileSync(join(dir, file), "utf8"));
+	}
+	return result;
+}
+
+describe("locale key parity", () => {
+	const en = loadLocale("en");
+	const de = loadLocale("de");
+
+	test("en and de have the same namespaces", () => {
+		expect(Object.keys(en).sort()).toEqual(Object.keys(de).sort());
+	});
+
+	for (const ns of Object.keys(en)) {
+		test(`${ns}: every en key exists in de`, () => {
+			const enKeys = flattenKeys(en[ns]);
+			const deKeys = flattenKeys(de[ns]);
+			const missing = enKeys.filter((k) => !deKeys.includes(k));
+			expect(missing).toEqual([]);
+		});
+
+		test(`${ns}: every de key exists in en`, () => {
+			const enKeys = flattenKeys(en[ns]);
+			const deKeys = flattenKeys(de[ns]);
+			const orphan = deKeys.filter((k) => !enKeys.includes(k));
+			expect(orphan).toEqual([]);
+		});
+
+		test(`${ns}: no empty de values`, () => {
+			const deKeys = flattenKeys(de[ns]);
+			for (const key of deKeys) {
+				const value = key
+					.split(".")
+					.reduce(
+						(obj, k) => (obj as Record<string, unknown>)?.[k],
+						de[ns] as unknown,
+					);
+				expect(
+					typeof value === "string" && value.length > 0,
+				).toBe(true);
+			}
+		});
+	}
+});