feat: per-instance terminal sessions for multi-instance switching

[linletian]

Overview

Refactor the frontend terminal from a single shared xterm.js instance to a per-instance terminal session model. Each running instance now owns its own terminal, transport state, timers, and reconnect logic.

Key Changes

Frontend Architecture

• Per-instance sessions: terminalSessions = {} stores independent session state for each instance
• Session lifecycle: New functions for create/destroy/reconcile session management
• Connection preservation: Switching tabs hides inactive terminals instead of tearing down healthy PTY connections
• TUI program protection: Long-lived TUI programs (Copilot CLI, vim, etc.) remain connected when switching away

New Functions

• createTerminalSession(id) - Create new session with dedicated xterm instance
• destroyTerminalSession(id) - Complete cleanup including termDataDisposable
• maybeDestroyInactiveStoppedSession(id) - Cleanup stopped non-active sessions
• reconcileTerminalSessions() - Sync sessions with instance state on refresh
• reconnectRunningTerminalSessions() - Auto-reconnect running sessions after network recovery
• renderTerminalSessions() - Show/hide session containers based on active selection
• reportSessionError(session, prefix, err) - Unified per-session error handling

Documentation Updates

• docs/API.md: Added frontend session model description
• docs/ARCHITECTURE.md: Updated §5.3 Instance Switch Protocol with 3 scenarios (stopped, running+healthy, running+unhealthy)
• docs/PRD.md: Added per-instance session behavior note

Tests

• TestIndexHTMLCoversMultiInstanceSwitching - Verify switching hooks
• TestIndexHTMLCoversSessionLifecycle - Verify create/destroy hooks
• TestIndexHTMLCoversReconcileLogic - Verify sync logic
• TestIndexHTMLCoversPerSessionConnectionManagement - Verify WebSocket management

Benefits

1. No more TUI disruption: Copilot CLI and other TUI programs stay connected when switching instances
2. Better resource management: Explicit disposable cleanup for onData subscriptions
3. Faster switching: Healthy sessions don't need full reconnect
4. Isolated state: Each instance has independent logCursor, ttyState, and terminal buffer

Testing

• All existing tests pass
• New tests verify session lifecycle and connection management
• Manual testing recommended for:
  • Switching between multiple running TUI instances
  • Network reconnection scenarios
  • Memory usage with many sessions

Checklist

• Code formatted with gofmt
• All tests pass
• Documentation updated
• Commit message follows conventions