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/i18n.rs

@@ -156,7 +156,7 @@ mod tests {
         ("file_picker.rs", include_str!("file_picker.rs")),
         ("geometry.rs", include_str!("geometry.rs")),
         ("library.rs", include_str!("library.rs")),
-        ("main.rs", include_str!("main.rs")),
+        ("app.rs", include_str!("app.rs")),
         ("page_export.rs", include_str!("page_export.rs")),
         ("pdf_doc.rs", include_str!("pdf_doc.rs")),
         ("pdf_merge.rs", include_str!("pdf_merge.rs")),