modularize date, database, and model code

- Add reusable date parsing, formatting, and recurrence modules
- Split database import helpers and model types into focused modules
- Expose the application as a library for the binary

src/ui.rs

@@ -11,9 +11,9 @@ use ratatui::{
 
 use crate::{
     app::{App, FormKind, InputKind, Mode, form_choices},
+    date::format_when,
     filter,
     model::{Aggregate, Item, ViewColumn, ViewKind},
-    parser::format_when,
 };
 
 #[derive(Clone, Copy)]