Init wizard: preserve field values when editing configured adapter

[Aaronontheweb]

Problem

When editing a previously configured adapter in the channel picker (via [e]), all fields start blank and must be re-entered from scratch. The VM preserves state, but the views don't pre-populate inputs on re-entry.

Affected Fields

Needs Termina patch (blocked on Aaronontheweb/termina#203)

• DM toggle (SelectionListNode) — _highlightedIndex is private, always starts at 0. Need WithHighlightedIndex(int) to pre-select Yes/No.
• Enable toggle — same issue, but skipped via SkipEnableSubStep in the picker flow.

Needs Termina investigation (password input + .Text setter)

• Bot token (Discord/Slack) — setting .Text on a password TextInputNode before focus causes the Submitted observable to fire immediately on paste, looping the user back to the same step. May need a Termina fix for .AsPassword() + .Text pre-seeding, or a different approach (e.g., showing masked placeholder text like •••••••• and only replacing on new input).
• App token (Slack) — same issue as bot token.

Already working

• Channel IDs (Discord), channel names (Slack), allowed user IDs (both), owner identity (Slack) — these use plain TextInputNode and pre-populate correctly today.

Context

Discovered during manual TUI testing of #763 (unified channel picker step). The pre-population attempt for password fields was reverted in that PR after it caused an input loop bug.

[Aaronontheweb]

Implementation note: A naive approach (setting .Text = vm.BotToken on the password TextInputNode before focus) was attempted and reverted — it caused an input loop where pasting a token kept redirecting back to the same sub-step. The Submitted observable appears to fire during the paste handling when .Text is pre-seeded, creating a re-entrancy issue.

Any fix here will need to account for the interaction between .AsPassword(), .Text pre-seeding, bracketed paste events, and the Submitted observable. Likely requires changes in Termina's TextInputNode rather than just consumer-side code.