northersubair: reusable landing view + empty state

[northersubair]

Extracts the landing view into a reusable component (#787) and adds a designed empty state (#785).

• Extract LandingPage into a reusable component #787 extract into a reusable component — new src/components/landing/landing-view.tsx owns the "render a lazily-loaded dashboard, or an empty state" branch. Navigation-style routes can now reuse it for any dashboard instead of duplicating the logic; src/app/page.tsx composes <LandingView view={SLADashboardView} />.
• Improve empty-state design for LandingPage #785 empty-state design — LandingView renders a designed, announced empty state (role="status") when isEmpty, and accepts a caller-supplied emptyState node. LandingEmptyState is exported for direct reuse.

Adds tests/landing-view.test.tsx (6 tests).

Scope note: the landing dashboard view (src/components/dashboard/sla-dashboard-view.tsx) is corrupted upstream (a 0-byte file) and is deliberately left untouched, per the issue-only approach. Tests mock next/dynamic so the corrupted module is never executed; the TS2306 ... is not a module error is pre-existing.

Closes #784
Closes #785
Closes #786
Closes #787

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