Fix synced client tunnel endpoints

[ZacharyZcR]

Summary

• carry each selected client-tunnel host's stable sync id alongside its local database id
• resolve that sync id to the corresponding host row on the remote Termix server
• migrate loaded local tunnel configs from their local host id when possible
• fail explicitly when the synced endpoint no longer exists instead of connecting to an unrelated same-numbered host

Verification

• npm test -- --run src/backend/tests/hosts/tunnel/c2s-source-host.test.ts
• npm run type-check
• npm run build
• targeted ESLint and Prettier checks

Fixes Termix-SSH/Support#1198