amberly-d: landing page i18n + memoization

[amberly-d]

Internationalizes the landing page loading state (#789) and memoizes the route (#791).

• Add i18n translations for LandingPage #789 i18n translations — the hard-coded "Loading dashboard..." string is replaced by t("dashboard.loading") via useI18n(), so the landing route follows the app locale. The fallback announces itself with role="status" / aria-live="polite".
• Memoize expensive computations in LandingPage #791 memoize expensive computations — the translated fallback (LandingLoading) and the route component are both wrapped in React.memo and defined at module scope, so their element types stay stable and the subtree is reused instead of re-created while the dashboard chunk loads.

Adds tests/landing-page-i18n-memo.test.tsx (4 tests).

Scope note: the landing dashboard view (src/components/dashboard/sla-dashboard-view.tsx) and the i18n message bundles are corrupted upstream (a 0-byte file and binary JSON respectively) and are deliberately left untouched, per the issue-only approach. Tests mock @/i18n/i18n and next/dynamic, so neither corrupted module is executed; the TS2306 ... is not a module error is pre-existing.

Closes #788
Closes #789
Closes #790
Closes #791

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