Add server name dropdown for saved connections

[erikdarlingdata]

Summary

• Replaces the AutoCompleteBox with a TextBox + dropdown button + Popup ListBox for the server name field in the connection dialog
• Users can now click the dropdown arrow to see all previously saved connections, sorted by most recently used
• Selecting a saved server fills in the name and applies stored settings (auth type, encryption, credentials)
• Users can still type a new server name directly in the text box

Closes #183

Test plan

• Open connection dialog, verify dropdown arrow is visible with white border matching other controls
• Click dropdown arrow, verify all saved connections appear in a popup list
• Select a saved connection, verify server name is filled and settings (auth, encryption, cert) are applied
• Type a new server name directly, verify test connection and connect still work
• Connect to a new server, reopen dialog, verify it appears in the dropdown

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