feat(session): persist and restore window size, maximized, and fullscreen state

[roramirez]

Summary

mmterm always reopened at the fixed config.window.width/height, discarding
however the window was actually left. This teaches session persistence to
remember the window geometry — maximized, fullscreen, or the last resized inner
size — and reapply it when the session is restored, for both the default
session and named --scope sessions.

Changes

• src/session/mod.rs — new SavedWindowState { maximized, fullscreen, width, height }
  and an optional window_state field on SavedSession. Uses
  #[serde(default, skip_serializing_if = "Option::is_none")] (same pattern as
  theme) so existing session files without the key still load cleanly.
• src/restore.rs — build_saved_session captures the live window's
  is_maximized(), fullscreen(), and inner_size(); yields None when there
  is no window.
• src/winit_handler.rs — resumed() loads the session once up front and
  applies fullscreen / maximized / inner size to the WindowAttributes before
  the window is mapped (avoids a visible post-map resize); the same loaded value
  seeds tab restore (no second load_from). Precedence: fullscreen > maximized > size.
• Tests — TOML round-trip, backward-compat (missing key → None), skip-when-none,
  save/load fullscreen round-trip, and a headless build_saved_session no-window case.
• CHANGELOG — entry under Added.

How to test

1. cargo run -- --scope demo, then maximize the window (or Alt+Enter for
   fullscreen) and/or resize it.
2. Quit and save the session: Ctrl+Q → s.
3. Relaunch cargo run -- --scope demo — the window reopens maximized /
   fullscreen / at the saved size.
4. An older session.toml lacking window_state still loads without error.
5. cargo test window_state covers the serialization matrix.