Confirm closing active host connections

[ZacharyZcR]

Summary

• Expose live connection state from terminal and Guacamole tabs through their tab refs
• Show one close confirmation per active host tab, with the host/tab name in the prompt
• Skip close confirmations for disconnected host tabs and add beforeunload protection while any host connection is still active

Fixes Termix-SSH/Support#857
Fixes Termix-SSH/Support#858

Verification

• npx prettier --check src/ui/AppShell.tsx src/ui/shell/tabUtils.tsx src/ui/features/terminal/Terminal.tsx src/ui/features/terminal/terminal-types.ts src/ui/features/guacamole/GuacamoleApp.tsx src/ui/features/guacamole/GuacamoleDisplay.tsx src/types/ui-types.ts src/types/index.ts
• npx eslint src/ui/AppShell.tsx src/ui/shell/tabUtils.tsx src/ui/features/terminal/Terminal.tsx src/ui/features/terminal/terminal-types.ts src/ui/features/guacamole/GuacamoleApp.tsx src/ui/features/guacamole/GuacamoleDisplay.tsx src/types/ui-types.ts src/types/index.ts
• npm run type-check
• git diff --check

Build note

• npm run build still fails before this change completes because src/ui/index.css cannot resolve @fontsource/jetbrains-mono/400.css in the current checkout.