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/friendly/strings/en-US.yaml

@@ -199,6 +199,11 @@ help:
                                  no snapshot is taken before each change
                                  (no per-command overhead), and undo/redo
                                  report that undo is turned off.
+            --mode <simple|advanced>
+                                 Start in this input mode, overriding the
+                                 project's stored mode. Without it, the
+                                 project's last-used mode is restored
+                                 (default: simple).
 
     App-level commands (typed inside the app, available in both modes):
         quit                     Exit cleanly.