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/app.css

@@ -401,3 +401,20 @@ html, body {
   color: var(--ctp-text);
   font-weight: 500;
 }
+
+/* ── Terminal section ─────────────────────────────────────── */
+.agent-messages {
+  flex: 1;
+  min-height: 0;
+  overflow-y: auto;
+  display: flex;
+  flex-direction: column;
+  gap: 0.375rem;
+}
+
+.terminal-section {
+  height: 12rem;
+  min-height: 8rem;
+  border-top: 1px solid var(--ctp-surface0);
+  flex-shrink: 0;
+}