entities: Spell gains optional level/description; demo spells

- Spell is no longer a name-only stub: optional level and description
  (serde skips missing fields, so old saved JSON stays parseable)
- mock: demo prepared spells — two vacant slots, Огненный шар (level 3,
  with description) and Магическая броня (level 1)

src/entities/mock.rs

@@ -184,10 +184,22 @@ fn generate_mock_contact_book () -> ContactBook {
 }
 
 
-/// Demo prepared spells. Currently an empty list — `Spell` is a stub.
+/// Demo prepared spells: a vacant slot, two known spells, one more
+/// vacant slot. The UI can fill and empty slots freely.
 fn generate_prepared_spells_list () -> PreparedSpells {
     let list = vec![
-
+        PreparedSpell::Vacant,
+        PreparedSpell::Occupied(Spell {
+            name: "Огненный шар".to_string(),
+            level: Some(3),
+            description: Some("Вспышка пламени в точке, радиус 6 м.".to_string()),
+        }),
+        PreparedSpell::Occupied(Spell {
+            name: "Магическая броня".to_string(),
+            level: Some(1),
+            description: None,
+        }),
+        PreparedSpell::Vacant,
     ];
 
     PreparedSpells(list)

src/entities/mod.rs

@@ -265,12 +265,13 @@ pub enum PreparedSpell {
     Occupied (Spell)
 }
 
-/// A spell. Only the name exists so far; the rest is a `@TODO`
-/// (see ROADMAP phase 2).
-// @TODO
-#[derive(Clone, Debug, Store, Serialize, Deserialize)]
+/// A spell: the name plus optional level and description.
+/// Optional fields keep old saved JSON (name-only spells) parseable.
+#[derive(Clone, Debug, Store, Serialize, Deserialize, Default)]
 pub struct Spell {
-    pub name: String
+    pub name: String,
+    pub level: Option<u8>,
+    pub description: Option<String>,
 }
 
 /// Hit dice: how many of the current die kind are already spent.