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/main.rs

@@ -1,16 +1,6 @@
-mod app;
-mod db;
-mod filter;
-mod macro_lang;
-mod model;
-mod parser;
-mod preferences;
-mod ui;
-
 use std::{io::stdout, path::PathBuf, time::Duration};
 
 use anyhow::{Context, Result};
-use app::App;
 use clap::Parser;
 use crossterm::{
     event::{self, Event, KeyEventKind},
@@ -18,8 +8,8 @@ use crossterm::{
     execute,
     terminal::{EnterAlternateScreen, LeaveAlternateScreen, disable_raw_mode, enable_raw_mode},
 };
-use db::{Database, Preset};
 use ratatui::{Terminal, backend::CrosstermBackend};
+use rogue_agenda::{App, Database, Preset, ui};
 
 #[derive(Debug, Parser)]
 #[command(