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/model/settings.rs

@@ -0,0 +1,36 @@
+use super::{DateOrder, DonePolicy, TrashPolicy, WeekStart};
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct DocumentSettings {
+    pub description: String,
+    pub backup_on_open: bool,
+    pub trash_policy: TrashPolicy,
+    pub done_policy: DonePolicy,
+    pub automatic_filing: bool,
+    pub date_order: DateOrder,
+    pub week_start: WeekStart,
+    pub default_time: String,
+    pub morning_time: String,
+    pub afternoon_time: String,
+    pub evening_time: String,
+    pub note_tab_width: u8,
+}
+
+impl Default for DocumentSettings {
+    fn default() -> Self {
+        Self {
+            description: String::new(),
+            backup_on_open: false,
+            trash_policy: TrashPolicy::OnDemand,
+            done_policy: DonePolicy::Keep,
+            automatic_filing: true,
+            date_order: DateOrder::YearMonthDay,
+            week_start: WeekStart::Monday,
+            default_time: "09:00".into(),
+            morning_time: "09:00".into(),
+            afternoon_time: "13:00".into(),
+            evening_time: "18:00".into(),
+            note_tab_width: 4,
+        }
+    }
+}