feat(web): integrate xterm.js with WebSocket terminal backend

- Install @xterm/xterm, @xterm/addon-fit, @xterm/addon-web-links
- Create useTerminal hook connecting to /terminal Socket.IO namespace
  with full session lifecycle management (create, input, resize, close)
- Create XTerminal component with FitAddon auto-resize, WebLinksAddon,
  theme support via CSS variables, and terminal:exit reconnect UI
- Replace mock TerminalPanel with real XTerminal component, preserving
  the tab bar and close button layout structure
- Export XTerminal and XTerminalProps from the terminal barrel index
- Write 40 unit tests covering useTerminal hook and both components

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/components/terminal/index.ts

@@ -1,2 +1,4 @@
-export type { TerminalLine, TerminalTab, TerminalPanelProps } from "./TerminalPanel";
+export type { TerminalTab, TerminalPanelProps } from "./TerminalPanel";
 export { TerminalPanel } from "./TerminalPanel";
+export type { XTerminalProps } from "./XTerminal";
+export { XTerminal } from "./XTerminal";