Start page and document tabs

Start page (shown on launch without a file, and from the house button in the application bar):
New Design, Open, recent designs (kept in the preferences, newest first) and built-in samples,
each card with a picture rendered from the design in the background. Samples are scripts of
engine commands: the sample plate plus a sheet metal bracket, a hinge assembly with a revolute
joint, a plastic enclosure (lip, bosses, rest, snap fit), a bolted flange with a parametric
circular pattern and a vent plate with a parametric rectangular pattern (test: all build
without a failing feature).

Several open designs as tabs in the application bar: each keeps its own session (undo,
selection), camera and hidden/shown items; + and Ctrl+N open a new design, opening a file or a
sample uses a new tab unless the current design is blank, closing a tab with unsaved changes
asks Save / Don't Save / Cancel. Control channel: ui.documents and ui.home (docs/control-protocol.md).

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

apps/solvecraft/src/main.rs

@@ -128,6 +128,12 @@ fn main() -> eframe::Result {
             for f in &files {
                 app.open_path(f);
             }
+            // No design given: the start page (not when driven over the control channel, whose
+            // clients expect the design view; `ui.home` shows it there).
+            if files.is_empty() && !sample && control_port.is_none() {
+                let rev = app.session.revision;
+                app.home.show(rev);
+            }
             Ok(Box::new(App(app)))
         }),
     )