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

docs/agents.md

@@ -491,7 +491,8 @@ not read or written.
 
 ## Seeing the UI
 
-To work on the UI, start the app with `cargo run -p effectcraft -- --control 9877` and use MCP bridge
+To work on the UI, start the app with `cargo run -p effectcraft -- --control 9877` (add `--demo` to
+open the demo project; without it the app starts with an empty project) and use MCP bridge
 mode or the raw control channel. A good loop is: `ui_elements` to find an id, `ui_click` or `ui_drag`
 to act, then `screenshot {"panel":"Timeline"}` to check the result. `render_frame` shows the
 composition itself at any zoom.