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.svelte

@@ -1,4 +1,6 @@
 <script lang="ts">
+  import Terminal from './Terminal.svelte';
+
   // ── Types ────────────────────────────────────────────────────
   type AgentStatus = 'running' | 'idle' | 'stalled';
   type MsgRole = 'user' | 'assistant' | 'tool-call' | 'tool-result';
@@ -165,16 +167,22 @@
               role="tabpanel"
               aria-label="Model"
             >
-              {#each project.messages as msg (msg.id)}
-                <div class="msg">
-                  <span class="msg-role {msg.role.split('-')[0]}">{msg.role}</span>
-                  <div
-                    class="msg-body"
-                    class:tool-call={msg.role === 'tool-call'}
-                    class:tool-result={msg.role === 'tool-result'}
-                  >{msg.content}</div>
-                </div>
-              {/each}
+              <div class="agent-messages">
+                {#each project.messages as msg (msg.id)}
+                  <div class="msg">
+                    <span class="msg-role {msg.role.split('-')[0]}">{msg.role}</span>
+                    <div
+                      class="msg-body"
+                      class:tool-call={msg.role === 'tool-call'}
+                      class:tool-result={msg.role === 'tool-result'}
+                    >{msg.content}</div>
+                  </div>
+                {/each}
+              </div>
+              <!-- Terminal section -->
+              <div class="terminal-section">
+                <Terminal />
+              </div>
             </div>
 
             <!-- Docs tab placeholder -->