Add a dark/light theme toggle

Summary

Following up on #2653 (dark theme redesign): once a dark theme lands as the default, add a toggle so users can switch between dark and light themes on demand instead of being locked into a single theme.

Proposal

• A toggle control in the navbar (visible on every page via the shared layout) to switch between dark and light modes.
• Persist the chosen theme per-browser (e.g. via localStorage) so it's remembered across sessions and page loads.
• Applies instantly without a full page reload where practical.
• Both themes share the same layout/markup - only the color tokens change, so this shouldn't require duplicating templates.

Motivation

Theme preference is subjective and environment-dependent (ambient lighting, personal preference, screen-sharing during a demo, etc.). Shipping only a dark theme solves the problem raised in #2653 for most users, but a toggle makes it work for everyone without forcing a choice.

Plan

This builds on top of the branch from #2653 (the toggle needs the dark theme tokens to exist first), so its PR will be opened against the same base and may land before or after #2653 merges.