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/en.ts

@@ -0,0 +1,27 @@
+// English source strings. Flat keys, dot-separated namespaces.
+// Add a locale by copying this file — keys must match exactly.
+export const en = {
+	"nav.board": "Board",
+	"nav.monitors": "Monitors",
+	"nav.domains": "Domains",
+	"nav.systems": "Systems",
+	"nav.incidents": "Incidents",
+	"nav.status": "Status",
+
+	"header.search": "Search (⌘K)",
+	"header.settings": "Settings",
+	"header.theme": "Toggle theme",
+	"header.addWidget": "Add widget",
+	"header.addService": "Add service",
+	"header.widget": "Widget",
+
+	"common.add": "Add",
+	"common.edit": "Edit",
+	"common.delete": "Delete",
+	"common.cancel": "Cancel",
+	"common.save": "Save",
+	"common.test": "Test",
+	"common.loading": "Loading…",
+} as const;
+
+export type Key = keyof typeof en;

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;
+}