Proposal: live PTY terminal pane with Cmd+Return fenced-code execution

## What

Terminal pane in the editor window (⌘⌥T), backed by SwiftTerm (MIT). Real PTY, bottom split, resizable/closable.

Cmd+Return with cursor in a ```bash/sh/zsh/shell``` fence runs that block directly in the pane (auto-opens if closed). Other languages/no tag: no-op.

Also: bundled MesloLGS NF font (Apache 2.0) + Paper/Dark ANSI themes, switchable in Preferences.

## Why

I read a lot of markdown docs with embedded shell snippets (setup steps, quick test commands) and kept alt-tabbing to a terminal to run them. Wanted to run the snippet where I'm already reading it, no context switch.

Prototype: `https://github.com/VinhTieuGit/macdown/tree/feature/terminal-pane`

## Tradeoffs

- New dependency: SwiftTerm.
- Bundled font adds binary size (few hundred KB).
- macOS only, no plans beyond that.

## Ask

Would love feedback on whether this fits the project's direction. Happy to open it as one PR, or split into smaller pieces (pane / font+theme / fence-exec) if that's easier to review. No worries either way if it's not a fit — just wanted to check before polishing further.

Review the linked feature/terminal-pane prototype first, then inspect MacDown’s editor and preference entry points to determine whether the pane, fenced-code execution, and font/theme work should be separate changes. Done would mean an agreed scope and maintainer decision before implementation.

Written by the indexing model from the issue text.