feat: another wave of features (#61)

## Explanation

This PR bundles several user-facing improvements and feature additions for the v0.3.0 release, including quality-of-life improvements to the UI, new slash commands, better state persistence, and auto-update checking.

## Included Changes

- **Resizable chat input** with drag handle (#58 partial)
- **Arrow key navigation fix** - cursor keys now navigate text when user has typed input (#58)
- **Scroll position persistence** per conversation tab
- **/skill command** for invoking Claude Code skills (#57)
- **Stats persistence fix** - stats now persist across session changes, only reset on disconnect (#59)
- **Auto-update checker** on startup (#17)
- **Resizable character panel** with full-height sprites (#10)
- **Font size and zoom settings** with keyboard shortcuts (Ctrl++/Ctrl+-/Ctrl+0) (#19)

## Closes

Closes #10, #17, #19, #57, #58, #59

## Attestations

- [x] I have read and agree to the Code of Conduct
- [x] I have read and agree to the Community Guidelines
- [x] My contribution complies with the Contributor Covenant
- [x] I have run the linter and resolved any errors
- [x] My pull request uses an appropriate title, matching the conventional commit standards
- [x] My scope of feat/fix/chore/etc. correctly matches the nature of changes in my pull request
- [x] All new and existing tests pass locally with my changes
- [x] Code coverage remains at or above the configured threshold

## Documentation

N/A - Internal app features

## Versioning

Minor - My pull request introduces new non-breaking features.

---
✨ This PR was created with help from Hikari~ 🌸

Co-authored-by: Hikari <hikari@nhcarrigan.com>
Reviewed-on: #61
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

src-tauri/src/config.rs

@@ -61,6 +61,15 @@ pub struct HikariConfig {
 
     #[serde(default)]
     pub always_on_top: bool,
+
+    #[serde(default = "default_update_checks_enabled")]
+    pub update_checks_enabled: bool,
+
+    #[serde(default)]
+    pub character_panel_width: Option<u32>,
+
+    #[serde(default = "default_font_size")]
+    pub font_size: u32,
 }
 
 impl Default for HikariConfig {
@@ -77,10 +86,17 @@ impl Default for HikariConfig {
             notifications_enabled: true,
             notification_volume: 0.7,
             always_on_top: false,
+            update_checks_enabled: true,
+            character_panel_width: None,
+            font_size: 14,
         }
     }
 }
 
+fn default_update_checks_enabled() -> bool {
+    true
+}
+
 fn default_greeting_enabled() -> bool {
     true
 }
@@ -93,6 +109,10 @@ fn default_notification_volume() -> f32 {
     0.7
 }
 
+fn default_font_size() -> u32 {
+    14
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, Default, PartialEq)]
 #[serde(rename_all = "lowercase")]
 pub enum Theme {
@@ -117,6 +137,9 @@ mod tests {
         assert!(config.greeting_enabled);
         assert!(config.greeting_custom_prompt.is_none());
         assert!(!config.always_on_top);
+        assert!(config.update_checks_enabled);
+        assert!(config.character_panel_width.is_none());
+        assert_eq!(config.font_size, 14);
     }
 
     #[test]
@@ -133,6 +156,9 @@ mod tests {
             notifications_enabled: true,
             notification_volume: 0.7,
             always_on_top: true,
+            update_checks_enabled: true,
+            character_panel_width: Some(400),
+            font_size: 16,
         };
 
         let json = serde_json::to_string(&config).unwrap();
@@ -143,7 +169,10 @@ mod tests {
         assert_eq!(deserialized.auto_granted_tools, config.auto_granted_tools);
         assert_eq!(deserialized.theme, Theme::Light);
         assert!(deserialized.greeting_enabled);
-        assert_eq!(deserialized.greeting_custom_prompt, Some("Hello!".to_string()));
+        assert_eq!(
+            deserialized.greeting_custom_prompt,
+            Some("Hello!".to_string())
+        );
     }
 
     #[test]