Summary

• allow the TUI text input to delegate Backspace at cursor start to the composer
• merge the previous backslash-continued composer line into the active input so Backspace deletes the continuation boundary
• add regression coverage for continuation-line merging

Fixes #18394

Tests

• npm test -- composerContinuationBackspace textInputLineNav
• npm run type-check
• git diff --check origin/main...HEAD