Simplify dark mode switcher and limit to home

[treyorr]

Summary:

• Keep theme preference logic in the root layout while moving the visible switcher to apps/web/src/routes/+page.svelte so it appears only on the / page
• Clean up dark mode handling to ensure consistent detection for users landing on arbitrary routes and leverage the expected Lucide Svelte icon package
• Documented behavior changes to clarify why the switcher is scoped to the home screen

Testing:

• Not run (not requested)