[P0] Match app background to whitepaper black

[kgarg2468]

Problem

The app background currently uses a white-to-green gradient, but product direction requires matching the whitepaper black background.

Evidence: app/globals.css sets body to:

linear-gradient(180deg, #FFFFFF 0%, #F5F5F5 15%, #0D1F1C 45%, #021a13 100%)

Scope

• Replace global page background with approved black background token/value.
• Ensure text contrast and card readability stay acceptable across app pages.

Acceptance Criteria

• Dashboard, Data, Simulate, and Report pages render with black background (no white top gradient).
• No readability regressions in key text/buttons.
• No flash of old background during route changes.

Suggested Files

• app/globals.css
• app/(app)/layout.tsx