fix: resolve SSH profile display on welcome screen

[datlechin]

Summary

• Connections using SSH profiles showed SSH : @ instead of SSH : user@host on the welcome screen because display code read the empty inline sshConfig instead of resolving the referenced SSH profile
• Added effectiveSSHConfig(profile:) on DatabaseConnection to centralize SSH config resolution (profile takes priority over inline config when sshProfileId is set)
• Fixed connectionSubtitle in WelcomeWindowView and ConnectionURLFormatter to use the resolved SSH config
• Fixed secondary bug: "Copy as URL" was loading SSH password from connection ID instead of profile ID when a profile was used
• Refactored DatabaseManager.buildEffectiveConnection to use the shared resolution method

Closes #454

Test plan

• 8 new tests pass (DatabaseConnectionSSHTests + ConnectionURLFormatterSSHProfileTests)
• Create a connection using an SSH profile → verify home screen shows SSH : user@host
• "Copy as URL" on SSH profile connection → verify URL contains correct SSH user/host
• Edit SSH profile (change username) → verify home screen reflects the change immediately
• Delete SSH profile → verify connection falls back gracefully