Add Navigate Back and Navigate Forward commands

[Copilot]

Adds two commands that drive the browser Navigation API from within em: Navigate Back (⌘ + [ / ⌘ + ←) and Navigate Forward (⌘ + ] / ⌘ + →), gated on whether history exists in that direction.

Commands

• New navigateBack / navigateForward commands. exec calls window.navigation.back() / forward(); canExecute reflects window.navigation.canGoBack / canGoForward, so each is disabled (greyed out in the Command Universe) with no history to navigate to or when the API is unavailable.

const navigateBackCommand: Command = {
  id: 'navigateBack',
  label: 'Navigate Back',
  keyboard: [{ key: '[', meta: true }, { key: Key.ArrowLeft, meta: true }],
  multicursor: false,
  svg: ArrowLeftIcon,
  canExecute: () => !!window.navigation?.canGoBack,
  exec: () => window.navigation?.back(),
}

Wiring

• Registered both in the src/commands/index.ts barrel and the Navigation group in COMMAND_GROUPS (constants.ts) — the latter is a runtime invariant (useCommandList throws otherwise).
• Reused ArrowLeftIcon / ArrowRightIcon; relaxed their FC<IconType> typing to the plain (props: IconType) => form used by all other command icons so they're assignable to Command.svg.

Types

• Declared a minimal window.navigation (canGoBack, canGoForward, back, forward) on the global Window interface, as the Navigation API is not yet in the TS DOM lib. Optional, reflecting unsupported browsers (e.g. older Safari).

Shortcut conflict

• ⌘ + ] was a secondary alias on categorize; removed it (primary ⌘ + Alt + O retained) and updated the categorize puppeteer test to use the primary shortcut.

Reviewer note

• The ⌘ + ← / ⌘ + → aliases preventDefault the native caret-to-line-start/end while editing. If undesirable, the arrow aliases can be dropped while keeping the bracket bindings.

[github-actions]

Vercel preview: https://em-peepafj56-cybersemics.vercel.app