feat(#3873): add dark mode styles to AppHeader, Dropdown, WorkSideMenu, and body

[twjeffery]

Summary

Four small Svelte changes so dark mode can target colours that are currently hardcoded.

Changes

1. reset.css body background. Changed #fff to reference --goa-color-greyscale-white.

2. Dropdown native chevron. Replaced the pseudo-element chevron (hardcoded #333 stroke) with an inline SVG that follows --goa-dropdown-color-text. Same size and position as before.

3. WorkSideMenu account popover background. Added --goa-work-side-menu-account-bg so dark mode can give the popover its own background. Token is added in the design-tokens package in feat(#3873): add work-side-menu-account-bg token design-tokens#154.

4. AppHeader and WorkSideMenu logos. Added dark variant SVGs and a MutationObserver on the data-theme attribute so logos swap between light and dark when the theme changes.

Note on the logo pattern

The logos use JS (a MutationObserver on the data-theme attribute) to swap the <img src> between light and dark variants. The rest of the dark mode story is pure CSS cascade. This inconsistency is intentional for now: multi-colour branded imagery doesn't convert cleanly to currentColor-based inline SVG, and a proper refactor would be significantly larger. Happy to discuss a cleaner pattern in a follow-up if the team prefers.

Context

Companion to #3850 (dark mode spike). The spike covered most of dark mode through token overrides. These changes address the shadow-DOM gaps the cascade can't reach.

How to test

1. npm install
2. npx nx serve react-prs
3. On the Everything page, Dropdowns section, the new native dropdown should look the same as before.
4. Logo swap: open devtools, run document.documentElement.setAttribute('data-theme', 'dark'). AppHeader and WorkSideMenu logos swap. Run with 'light' to swap back.

Fixes #3873