fix(tui): let command-backspace delete preceding newlines

[daviddanialy]

Summary

• Delete the preceding newline when Command–Backspace is pressed at the start of a logical line, allowing repeated presses to continue through earlier lines.
• Apply the same behavior to Ctrl–U, which shares the editor operation.
• Add regressions for repeated deletion, blank lines, Unicode, an empty buffer, preserved text after the cursor, and both key bindings.

Validation

• User manually verified the local TUI build.
• mise run test -- --lib tui:: -- --test-threads=1: 481 passed, 1 ignored in an isolated worktree.
• mise run lint: passed (formatting and Clippy with all features and no default features).
• git diff --check: passed.

The parallel TUI suite encounters an existing startup-test failure (catalog_returns_canonical_workspace_root); the same failure was reproduced on unmodified main. The serial patched suite passes. An initial parallel run also failed catalog_invalid_root_preserves_diagnostic, which passed in isolation.

Unrelated local changes to src/tui/mod.rs are excluded.

[kit-code-agent]

No issues found. The changes look good to merge.