tui: improve multiline composer

[bug-ops]

Summary

• cap the TUI composer at 3 visible lines while keeping multiline drafts scrollable
• add Ctrl+J as an additional newline chord alongside Shift+Enter
• update help text, docs, changelog, and zeph-tui coverage for the composer behavior

Validation

• cargo test -p zeph-tui --lib
• cargo +nightly fmt --check
• cargo clippy --all-targets --all-features --workspace -- -D warnings (blocked by missing nvcc for cudarc/candle-kernels in this environment)
• cargo nextest run --workspace --all-features --exclude exarch-python --exclude exarch-node --lib --bins (blocked by missing nvcc for cudarc/candle-kernels in this environment)