isInputElement does not recognize contenteditable="plaintext-only"

[magicismight]

Description

isInputElement() in manager.utils.ts does not recognize elements with contenteditable="plaintext-only" as input elements, causing hotkeys with ignoreInputs: true (the default for single keys like Backspace, Delete, etc.) to fire when the user is typing in such elements.

Reproduction

1. Register a single-key hotkey (e.g. Backspace) with default options (ignoreInputs defaults to true)
2. Focus a <div contenteditable="plaintext-only"> element
3. Press Backspace — the hotkey handler fires unexpectedly, in addition to the normal text editing behavior

Root Cause

The isInputElement function checks contentEditable property against 'true' and '', but not against 'plaintext-only':

// packages/hotkeys/src/manager.utils.ts
if (element instanceof HTMLElement) {
  const contentEditable = element.contentEditable
  if (contentEditable === 'true' || contentEditable === '') {
    return true
  }
}

The contenteditable="plaintext-only" attribute causes the DOM property element.contentEditable to return "plaintext-only", which doesn't match either check.

Suggested Fix

Use element.isContentEditable (a computed boolean that returns true for all editable states including plaintext-only and inherited contenteditable):

if (element instanceof HTMLElement && element.isContentEditable) {
  return true
}

This would also cover the edge case of inherited contenteditable (where element.contentEditable === "inherit" but element.isContentEditable === true).

Context

contenteditable="plaintext-only" is a valid HTML attribute value supported in all modern browsers. It's increasingly common in rich text editors and custom input components that want plain-text editing without formatting.