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.

src/action.rs

@@ -129,4 +129,11 @@ pub enum Action {
     /// refreshes the table list + schema cache.
     Undo,
     Redo,
+    /// User changed the input mode mid-session (the `mode` command).
+    /// The runtime records it through the worker so `project.yaml`
+    /// reflects the live mode and it is restored on the next open
+    /// (ADR-0015 mode-restore amendment, issue #14). Best-effort:
+    /// a persistence failure here must not escalate a UI mode toggle
+    /// into a fatal — the in-memory mode has already changed.
+    PersistMode(crate::mode::Mode),
 }