fix: resolve SSH tunnel endpoints reliably

[ZacharyZcR]

Summary

• resolve tunnel endpoint hosts by id, name, IP, or username@IP on both client and server paths
• allow local/dynamic tunnels to use ordinary target addresses without forcing a database endpoint host lookup
• keep remote tunnels strict when an endpoint SSH host is required
• align HostEditor manual tunnel starts with the normalized tunnel name format and endpoint credentials
• preserve autostart behavior for database endpoint hosts while allowing direct local targets

Refs Termix-SSH/Support#866

Validation

• npm run type-check
• npx eslint src/backend/ssh/tunnel.ts src/ui/features/tunnel/TunnelTab.tsx src/ui/features/tunnel/tunnel-endpoints.ts src/ui/sidebar/HostEditor.tsx
• npx prettier --check src/backend/ssh/tunnel.ts src/ui/features/tunnel/TunnelTab.tsx src/ui/features/tunnel/tunnel-endpoints.ts src/ui/sidebar/HostEditor.tsx
• git diff --check

Note: targeted vitest was not run because node_modules/.bin/vitest is not installed in this checkout.