feat: email-based collaboration invitations

[9trocode]

Summary

This PR adds email-based collaboration invitations, allowing terminal owners to share terminals with other users by email address. It also includes several bug fixes and improvements to the shared terminal experience.

Key Features

• Email-based invitations: Terminal owners can invite collaborators by email address
• Real-time status updates: Collaborators now see accurate agent online/offline status
• Improved UI: Fixed layout glitches on shared terminal cards

Bug Fixes

• Fixed shared terminals showing "Offline" instead of "Connect" button for collaborators
• Fixed UI alignment/layout issues with shared terminal cards (text overflow, date breaking, etc.)
• Agent status broadcasts now include collaborators, not just owners

Other Changes

• Added session duration options (default 24 hours)
• Support LXC/PVE environments without sudo in rexec-agent
• Added agent uninstall documentation
• Filter mouse-enable escape sequences to preserve browser scrolling

Files Changed

• Backend: internal/api/handlers/agent.go, container.go, container_events.go, collab.go, internal/storage/postgres.go
• Frontend: Dashboard.svelte, CollabPanel.svelte, Header.svelte, collab.ts, terminal.ts
• Agent: cmd/rexec-agent/main.go, scripts/install-agent.sh
• Docs: docs/AGENTS.md, AgentDocs.svelte

Testing

• Backend builds successfully (make build)
• Frontend builds successfully (npm run build)
• All Go tests pass