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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CONTRIBUTING.md

@@ -31,6 +31,8 @@ DASH_DEV=1 go run ./cmd/dash   # API on :8080
 - No auth, no YAML-as-source-of-truth, no external database. These are
   non-goals for v1.
 - Standard library before dependencies; existing helpers before new ones.
+- User-visible strings go through `t("key")` from `src/i18n` — add the key to
+  `en.ts` rather than inlining literals in new UI.
 
 ## Before pushing