DX: surface pending translations in the sidebar during dev

[greatest0fallt1me]

Summary

Discovering an untranslated key meant reading `lib/i18n/messages.ts` directly, or catching a dev-only `console.warn` at the exact moment `t()` happened to hit it at runtime -- no proactive way to see what's outstanding.

Change

• `lib/i18n/pendingTranslations.ts`: `getPendingTranslations(locale)` returns the keys present in `en` (the source of truth) but missing from that locale's own partial dictionary; `getAllPendingTranslations()` does this for every non-default locale.
• `components/Sidebar.tsx`: renders a small `role="status"` notice with the pending count per locale (e.g. `es: 4 pending`), right where a contributor is already looking. Gated on `NODE_ENV !== "production"` and hidden along with the rest of the nav content when the sidebar is collapsed.

Tests

• `lib/i18n/pendingTranslations.test.ts`: no pending keys for the default locale, correct pending set for `es`, every pending key is real, `getAllPendingTranslations` covers exactly the non-default locales and matches the single-locale function.
• `components/Sidebar.test.tsx`: notice renders with the expected text outside production, is hidden in production, and is hidden when the sidebar is collapsed.

`npx vitest run lib/i18n/pendingTranslations.test.ts components/Sidebar.test.tsx`: 12 passed. `npx tsc --noEmit` / `npx eslint` on changed files: clean.

Related to #113