feat: resizable character panel and full-height sprite

- Change sprite to use full height instead of full width for better scaling
- Add draggable divider between character panel and terminal
- Persist panel width preference in config (min: 200px, max: 600px)

Closes #10

src-tauri/src/config.rs

@@ -64,6 +64,9 @@ pub struct HikariConfig {
 
     #[serde(default = "default_update_checks_enabled")]
     pub update_checks_enabled: bool,
+
+    #[serde(default)]
+    pub character_panel_width: Option<u32>,
 }
 
 impl Default for HikariConfig {
@@ -81,6 +84,7 @@ impl Default for HikariConfig {
             notification_volume: 0.7,
             always_on_top: false,
             update_checks_enabled: true,
+            character_panel_width: None,
         }
     }
 }
@@ -126,6 +130,7 @@ mod tests {
         assert!(config.greeting_custom_prompt.is_none());
         assert!(!config.always_on_top);
         assert!(config.update_checks_enabled);
+        assert!(config.character_panel_width.is_none());
     }
 
     #[test]
@@ -143,6 +148,7 @@ mod tests {
             notification_volume: 0.7,
             always_on_top: true,
             update_checks_enabled: true,
+            character_panel_width: Some(400),
         };
 
         let json = serde_json::to_string(&config).unwrap();