feat(co-browser): Create Express routes and interactive HTML UI

[coderooz]

Create the HTTP layer for the co-browser system: Express routes for serving the UI and SSE viewport streaming.

Acceptance criteria:

• Create src/domains/browser/browser-window.js (Express router)
• Create public/co-browser.html (two-section interactive UI)
• Route: GET /browser/view/:windowId — serve HTML UI
• Route: GET /browser/stream/:windowId/:tabId — SSE viewport screenshots
• Route: POST /browser/instructions/:windowId — update instruction panel
• Route: GET /browser/state/:windowId — get browser state
• Mount routes in server.js
• HTML UI: left panel = live viewport, right panel = instructions, top = tab bar
• Tab bar shows lock icons for agent-owned tabs
• User can interact with the viewport (click, type, scroll)
• Instructions panel is static, markdown-rendered
• SSE streams screenshots at 500ms intervals for active tab
• Minimal CSS, no frameworks

Depends on: co-browser tools (#47)
Milestone: v2.8.0