Add window.postMessage interface for command add/delete from Command Hub

[Copilot]

Enables Selection Command Hub (and other trusted web pages) to programmatically add or delete extension commands via window.postMessage, with toast feedback on success/error.

Changes

New IPC command

• Added BgCommand.removeCommand enum value to ipc.ts

Background handler (background_script.ts)

• Added removeCommand handler: filters commands by ID in a single pass, returns false if ID not found, persists and returns true on success

postMessage listener (DownloadButton.tsx)

• Added window.message listener scoped to Hub origin (new URL(HUB_URL).origin) — rejects messages from any other origin
• Handles two actions:
  • AddCommand — delegates to existing BgCommand.addCommand
  • DeleteCommand — calls new BgCommand.removeCommand with command id
• Shows sonner toast on success/error; added <Toaster> to component render tree

i18n

• Added 4 keys (commandHub_add_success, commandHub_add_error, commandHub_delete_success, commandHub_delete_error) to all 14 supported locales

Usage (Hub side)

// Add a command
window.postMessage({ action: "AddCommand", command: JSON.stringify(cmd) }, "*")

// Delete a command by ID
window.postMessage({ action: "DeleteCommand", id: "command-id" }, "*")