feat: i18n seam — typed t() + en dictionary

Nav and header wired as proof. Full string extraction deferred until a
second locale exists; CONTRIBUTING records the convention.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/frontend/src/i18n/index.ts

@@ -0,0 +1,13 @@
+import { en, type Key } from "@/i18n/en";
+
+export type { Key } from "@/i18n/en";
+
+// i18n seam: dictionaries are plain objects keyed by dotted names. `t` is a
+// typed lookup that falls back to the key itself, so a missing translation
+// renders the key — loud in dev, never a crash. Locales beyond `en` plug in
+// here later (settings key + lazy import); nothing else changes.
+const dict: Record<Key, string> = en;
+
+export function t(key: Key): string {
+	return dict[key] ?? key;
+}