Add back affordance to channel editor

[edwin-zvs]

Summary

• show < back at the top-left of the TUI channel configuration editor
• make the affordance clickable while preserving Esc navigation
• record the behavior in the service-view design spec

Verification

• cargo test -p construct-cli service_channel_editor
• cargo test -p construct-cli service_view_renders_definition_help_channels_and_sessions
• cargo check -p construct-cli
• cargo build
• git diff --check

Visual artifact

A live VHS capture was attempted, but the isolated TUI auto-opened its Operator/configure focus surfaces and did not reliably reach the channel editor. I left it unattached rather than post a misleading demo. The new TestBackend test verifies both the rendered < back label and mouse navigation.