Popup + options pages don't match web UI's dark cyan/magenta theme

[paperhurts]

Observation

The web UI (`packages/web/`) is dark-themed with cyan/magenta accents per `spec.md`. The extension popup and options pages are light-themed (white-ish background, dark text, dark button). They look like two different products.

Source of truth:

• `packages/extension-shared/src/popup.html` — inline `<style>` block, `background: #fafafa`, `color: #1a1a1a`, button `#222`
• `packages/extension-shared/src/options.html` — same pattern

Proposed fix

Bring the extension pages in line with the web UI's palette:

• Dark background (whatever the web UI's body bg is — pull the exact token to avoid drift)
• Cyan + magenta accents matching the web UI's primary/secondary
• Same font family + sizing scale

Two ways to ship this without re-architecting:

1. Hand-port the small token set. The extension pages are tiny — popup is ~30 lines of CSS. Just copy the relevant CSS custom properties from the web UI and apply them. Lowest blast radius.
2. Extract shared CSS tokens into `extension-shared` or a new `@gitmarks/ui-tokens` package. More machinery, but right move if the popup/options pages grow.

I lean (1) for now — defer the token package until there's a third surface that needs it.

Out of scope

• Adding Tailwind to the extension (overkill for two ~30-line stylesheets)
• Redesigning the popup layout (separate concern)

Related

• Popup save button stuck in 'saving…' state with progress cursor after successful save #43 (popup save-button stuck state) — worth fixing in the same PR if we're already in popup.html/popup.ts