refactor(project): transition to library structure

Move module declarations from src/main.rs to a new src/lib.rs to transition
the project to a library-first structure. This allows other tools and
examples to utilize the core logic.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (gemma-4-31B-it-UD-Q4_K_XL.gguf)

src/lib.rs

@@ -0,0 +1,9 @@
+pub mod command;
+pub mod draw;
+pub mod format;
+pub mod formula;
+pub mod import;
+pub mod model;
+pub mod persistence;
+pub mod ui;
+pub mod view;