fix(theme): force light mode on marketing routes #3891

Originally written by @ccantynz-alt on GitHub.
Imported from https://github.com/ccantynz-alt/MarcoReid.com/pull/52

You're right that I should have caught this during the audit. Marketing pages were designed for the light palette only — every text colour assumes a white / navy-50 section background. When dark mode is on, sections without an explicit bg-* inherit the body's navy-950 background, and navy-700 text becomes navy-700 on navy-950 — unreadable. The screenshot of the accounting "numbers speak for themselves" section was the obvious example; the same class of bug is on every marketing page that has a section without a forced background.

Two-part fix so there's no flash either:

• Root layout inline theme script (runs before render) now checks the pathname. If it is NOT a platform route (/dashboard, /admin, /clients, /matters, /documents, /trust, /billing, /time, /voice, /news, /settings, /onboarding), it forces class="light" on immediately. Platform routes still respect the user's saved theme.
• Marketing layout adds a small client component (ForceLightTheme) as belt-and-braces — strips dark / adds light on mount, restores on unmount so platform pages still go dark when the user navigates back into the app.

This lets us ship now without doing a full dark-variant pass on every marketing page. When we do that pass, we can drop the override.