fix: Per-agent terminal isolation, floating terminal z-index, and char-by-char input

- Add :key to PromptBar to force remount on agent switch, fixing shared terminal session bug
- Raise AgentTerminal z-index above PromptBar backdrop so floating terminal is visible/clickable
- Send prompt text char-by-char (15ms delay) matching FloatingVoice pattern for Claude Code compat
- Guard xterm dispose against unloaded addons to prevent errors on agent switch
- Widen PromptBar panel from 360px to 420px to fit all ChatInput buttons

.claude/ui.json

@@ -5,5 +5,5 @@
   "gradient": "linear-gradient(135deg, #6366f1, #8b5cf6)",
   "terminalBg": "#0f0a1a",
   "terminalBorder": "#6366f1",
-  "enabled": false
+  "enabled": true
 }