refactor: move the event loop into the library as app::run (prep for Issue #42)

src/main.rs is now only the desktop entry point; the app itself lives in
src/app.rs, so other entry points such as android_main can start it. The code
is unchanged apart from crate:: paths; the source scans in the i18n and UI
tests and the docs point to app.rs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui.rs

@@ -6133,7 +6133,7 @@ mod tests {
     const UI_SOURCES: [(&str, &str); 4] = [
         ("ui.rs", include_str!("ui.rs")),
         ("selection.rs", include_str!("selection.rs")),
-        ("main.rs", include_str!("main.rs")),
+        ("app.rs", include_str!("app.rs")),
         ("file_picker.rs", include_str!("file_picker.rs")),
     ];