feat: persist & restore per-project input mode (#14)

The input mode always started in simple; a learner who quit in advanced
had to re-toggle every launch. Store the mode per-project in project.yaml
(project.mode:, optional, default simple) and restore it on every open.

Mode is live UI state, not schema: the worker stamps the current mode
into project.yaml on every write, so a later command rewrites the live
value rather than clobbering it — no db round-trip needed. The mode is
persisted on unload (quit + project switch) so the mode you leave a
project in is always what reopens; the `mode` command also persists
immediately. A switch saves the outgoing mode, then restores the
incoming project's stored mode.

New --mode simple|advanced CLI flag (precedence --mode > stored >
simple; combines with --resume). A teacher can ship a project that
opens in advanced mode and export it to students (the mode travels in
the zip).

ADR-0015 Amendment 1; ADR-0003 note; help banner; requirements L1b.

tests/iteration4b_lifecycle_commands.rs

@@ -304,6 +304,7 @@ fn project_switched_event_updates_state() {
         display_name: "New Name".to_string(),
         is_temp: false,
         history_entries: Vec::new(),
+        mode: rdbms_playground::mode::Mode::Simple,
     });
     assert_eq!(app.project_name.as_deref(), Some("New Name"));
     assert!(!app.project_is_temp);