feat(mmterm): auto-save session on SIGTERM/SIGHUP/SIGINT

[roramirez]

Summary

When mmterm is terminated unattended — PC shutdown/reboot (SIGTERM from systemd), logout / display-server teardown (SIGHUP), or kill (SIGINT) — the session was lost. Persistence only ran through the interactive Ctrl+Q / window-close prompt (InputMode::QuitSave), which needs a human keypress.

This adds signal handling so the session is auto-saved on those signals and can be restored on the next launch. Reopening with the same --scope restores the tabs, panes, layout, cwds, and theme, because the save routes through the existing scope-aware session_path().

Changes

• Cargo.toml — add signal-hook under [target.'cfg(unix)'.dependencies].
• src/main.rs — new App.shutdown_requested: Arc<AtomicBool> flag; spawn_signal_watcher() runs a background thread on Signals::new([SIGTERM, SIGHUP, SIGINT]), sets the flag, and nudges the event loop awake via the existing EventLoopProxy; wired in main() on Unix.
• src/winit_handler.rs — user_event checks the flag first; if set it saves and exits without prompting (an unattended shutdown must not block).
• src/restore.rs — save_session_on_shutdown() reuses build_saved_session() + session::save_to(), gated on general.restore_session.
• CHANGELOG.md — entry under [Unreleased] → Added.
• Tests in src/restore_test.rs.

Design notes: the actual save happens on the main thread (safe to touch App/self.window, no async-signal-safety constraints); the watcher thread only flips a flag and wakes the loop. restore_session = false still exits promptly, just without saving. Interactive quit paths are unchanged.

How to test

Automated:

cargo fmt --check
cargo clippy --locked -- -D warnings
cargo test

New tests: save_session_on_shutdown_{writes_when_restore_enabled,is_noop_when_restore_disabled,honors_scope_path}.

Manual (end-to-end):

1. cargo run -- --scope worktest, open a couple of tabs / splits, cd somewhere.
2. From another shell: kill -TERM $(pgrep -n mmterm) (repeat with -HUP, -INT).
3. Confirm ~/.config/mmterm/sessions/worktest.toml was written/updated.
4. cargo run -- --scope worktest → tabs, layout, cwds, theme restored.
5. Verify the interactive Ctrl+Q prompt and normal quit are unchanged.