feat: three-state dark mode toggle (light / system / dark)

[gregoryfoster]

Summary

The current dark mode toggle (added in #19) writes `pm-color-scheme` as either `"dark"` or `"light"` on every click. Once a user clicks the toggle, there is no UI path to return to the `"absent"` state (follow OS `prefers-color-scheme`). Users must manually clear `localStorage` via DevTools to restore system-following behavior.

Current behavior

localStorage value	Behavior
`"dark"`	Force dark
`"light"`	Force light
absent	Follow OS

The toggle cycles between `"dark"` ↔ `"light"` only. `"absent"` is never written.

Proposed behavior

Add a third state to the toggle: light → system → dark → light (or a dedicated reset affordance). Writing `"system"` (or clearing the key) to localStorage would return to OS-following mode.

Scope

• Update `dark-mode.js` to support a three-state cycle or explicit reset
• Update FOUC prevention script to handle a `"system"` stored value (treat as absent)
• Update STYLE.md section 3 localStorage key table and toggle button documentation
• Add tests

References

• Identified during code review of docs: create project style guide (docs/STYLE.md) #19
• STYLE.md section 3 documents the `pm-color-scheme` key values