feat(electrobun): add xterm.js terminal with image addon (Sixel/iTerm2)

- Terminal.svelte component with @xterm/xterm + Canvas + Fit + Image addons
- Catppuccin Mocha terminal theme matching main app
- Sixel, iTerm2 inline image protocol support via xterm-addon-image
- ResizeObserver for responsive terminal sizing
- Demo cargo test output in terminal section below agent messages

ui-electrobun/src/mainview/main.ts

@@ -1,4 +1,5 @@
 import "./app.css";
+import "@xterm/xterm/css/xterm.css";
 import App from "./App.svelte";
 import { mount } from "svelte";