Minor: persist passwords and enter-to-open tables

[davidbudnick]

Context

• Connection passwords were stripped on every config save, so reconnects after restart sent an empty password and failed auth.
• Sidebar j/k on the data/structure pane auto-opened the highlighted table and stole focus; open should require enter.

Testing

• go test ./internal/db/ ./internal/ui/ -count=1
• make lint (go vet ./...)
• Manual: Phoenix Engines connect with stored password; sidebar j/k keeps focus until enter opens data.