App: launching opens an empty project, not the demo (#204)

The desktop app opened the demo project whenever it wasn't given a
project file, so File ▸ New Composition on the Home screen added "Comp
4" to the three demo comps. As in After Effects, a launch now starts
with an empty Untitled Project. --demo opens the demo (Help ▸ Open
Demo Project and the Home screen tile still do); --empty is kept.

The arguments are parsed by a function now, with a test
(no_arguments_start_an_empty_project).

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_018gnTnH3Jmk5zr4tMq1Mc9f

README.md

@@ -193,7 +193,7 @@ To build it yourself you need [Rust](https://rustup.rs/) 1.95 or newer.
 ```sh
 git clone https://github.com/storytold/effectcraft
 cd effectcraft
-cargo run --release -p effectcraft          # the app, with the demo project open
+cargo run --release -p effectcraft          # the app (add `-- --demo` to open the demo project)
 cargo run --release -p effectcraft-cli -- render --out intro.mp4    # render the demo headless
 cargo xtask web --serve 8765                # the browser build on http://127.0.0.1:8765/ (docs/web.md)
 ```