refactor: consolidate TuiError into ChannelError

[bug-ops]

Context

zeph-tui::TuiError duplicates variants from zeph-core::ChannelError (Io, ChannelClosed, ConfirmCancelled). These are semantically identical.

Proposed change

• Remove TuiError from zeph-tui
• Use zeph_core::ChannelError directly in TUI channel impl
• Ensure zeph-channels::ChannelError has proper From impl for zeph-core::ChannelError

Affected crates

zeph-tui, zeph-channels, zeph-core

Complexity

Low

References

.local/plan/architecture-audit.md — Phase 1, item 1.2