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>

README.md

@@ -154,8 +154,9 @@ New code comments and commit messages are written in English; older code and his
 
 ```text
 src/
-├── main.rs         # Event loop (winit), mouse, touch and pen input, control flow
-├── lib.rs          # Library exports (for tests)
+├── main.rs         # Desktop entry point, starts app::run
+├── app.rs          # Event loop (winit), mouse, touch and pen input, control flow
+├── lib.rs          # Library exports (app and modules, also for tests)
 ├── canvas.rs       # wgpu rendering: paper, patterns, images, ink, neighboring pages
 ├── shader.wgsl     # Vertex and fragment shaders
 ├── viewport.rs     # Camera (zoom, pan), page stack of the continuous view, swipe momentum