Add built-in theme support with various pre-defined themes

- Introduce multiple built-in themes (`default_dark`, `default_light`, `gruvbox`, `dracula`, `solarized`, `midnight-ocean`, `rose-pine`, `monokai`, `material-dark`, `material-light`).
- Implement theming system with customizable color schemes for all UI components in the TUI.
- Include documentation for themes in `themes/README.md`.
- Add fallback mechanisms for default themes in case of parsing errors.
- Support custom themes with overrides via configuration.

crates/owlen-core/src/storage.rs

@@ -45,10 +45,7 @@ impl StorageManager {
         // Ensure the directory exists
         if !sessions_dir.exists() {
             fs::create_dir_all(&sessions_dir).map_err(|e| {
-                Error::Storage(format!(
-                    "Failed to create sessions directory: {}",
-                    e
-                ))
+                Error::Storage(format!("Failed to create sessions directory: {}", e))
             })?;
         }
 
@@ -66,7 +63,11 @@ impl StorageManager {
     }
 
     /// Save a conversation to disk
-    pub fn save_conversation(&self, conversation: &Conversation, name: Option<String>) -> Result<PathBuf> {
+    pub fn save_conversation(
+        &self,
+        conversation: &Conversation,
+        name: Option<String>,
+    ) -> Result<PathBuf> {
         self.save_conversation_with_description(conversation, name, None)
     }
 
@@ -75,7 +76,7 @@ impl StorageManager {
         &self,
         conversation: &Conversation,
         name: Option<String>,
-        description: Option<String>
+        description: Option<String>,
     ) -> Result<PathBuf> {
         let filename = if let Some(ref session_name) = name {
             // Use provided name, sanitized
@@ -101,26 +102,22 @@ impl StorageManager {
             save_conv.description = description;
         }
 
-        let json = serde_json::to_string_pretty(&save_conv).map_err(|e| {
-            Error::Storage(format!("Failed to serialize conversation: {}", e))
-        })?;
+        let json = serde_json::to_string_pretty(&save_conv)
+            .map_err(|e| Error::Storage(format!("Failed to serialize conversation: {}", e)))?;
 
-        fs::write(&path, json).map_err(|e| {
-            Error::Storage(format!("Failed to write session file: {}", e))
-        })?;
+        fs::write(&path, json)
+            .map_err(|e| Error::Storage(format!("Failed to write session file: {}", e)))?;
 
         Ok(path)
     }
 
     /// Load a conversation from disk
     pub fn load_conversation(&self, path: impl AsRef<Path>) -> Result<Conversation> {
-        let content = fs::read_to_string(path.as_ref()).map_err(|e| {
-            Error::Storage(format!("Failed to read session file: {}", e))
-        })?;
+        let content = fs::read_to_string(path.as_ref())
+            .map_err(|e| Error::Storage(format!("Failed to read session file: {}", e)))?;
 
-        let conversation: Conversation = serde_json::from_str(&content).map_err(|e| {
-            Error::Storage(format!("Failed to parse session file: {}", e))
-        })?;
+        let conversation: Conversation = serde_json::from_str(&content)
+            .map_err(|e| Error::Storage(format!("Failed to parse session file: {}", e)))?;
 
         Ok(conversation)
     }
@@ -129,14 +126,12 @@ impl StorageManager {
     pub fn list_sessions(&self) -> Result<Vec<SessionMeta>> {
         let mut sessions = Vec::new();
 
-        let entries = fs::read_dir(&self.sessions_dir).map_err(|e| {
-            Error::Storage(format!("Failed to read sessions directory: {}", e))
-        })?;
+        let entries = fs::read_dir(&self.sessions_dir)
+            .map_err(|e| Error::Storage(format!("Failed to read sessions directory: {}", e)))?;
 
         for entry in entries {
-            let entry = entry.map_err(|e| {
-                Error::Storage(format!("Failed to read directory entry: {}", e))
-            })?;
+            let entry = entry
+                .map_err(|e| Error::Storage(format!("Failed to read directory entry: {}", e)))?;
 
             let path = entry.path();
             if path.extension().and_then(|s| s.to_str()) != Some("json") {
@@ -172,9 +167,8 @@ impl StorageManager {
 
     /// Delete a saved session
     pub fn delete_session(&self, path: impl AsRef<Path>) -> Result<()> {
-        fs::remove_file(path.as_ref()).map_err(|e| {
-            Error::Storage(format!("Failed to delete session file: {}", e))
-        })
+        fs::remove_file(path.as_ref())
+            .map_err(|e| Error::Storage(format!("Failed to delete session file: {}", e)))
     }
 
     /// Get the sessions directory path
@@ -237,7 +231,9 @@ mod tests {
         #[cfg(target_os = "macos")]
         {
             // macOS should use ~/Library/Application Support
-            assert!(path.to_string_lossy().contains("Library/Application Support"));
+            assert!(path
+                .to_string_lossy()
+                .contains("Library/Application Support"));
         }
 
         println!("Default sessions directory: {}", path.display());
@@ -257,10 +253,13 @@ mod tests {
 
         let mut conv = Conversation::new("test-model".to_string());
         conv.messages.push(Message::user("Hello".to_string()));
-        conv.messages.push(Message::assistant("Hi there!".to_string()));
+        conv.messages
+            .push(Message::assistant("Hi there!".to_string()));
 
         // Save conversation
-        let path = storage.save_conversation(&conv, Some("test_session".to_string())).unwrap();
+        let path = storage
+            .save_conversation(&conv, Some("test_session".to_string()))
+            .unwrap();
         assert!(path.exists());
 
         // Load conversation
@@ -280,7 +279,9 @@ mod tests {
         for i in 0..3 {
             let mut conv = Conversation::new("test-model".to_string());
             conv.messages.push(Message::user(format!("Message {}", i)));
-            storage.save_conversation(&conv, Some(format!("session_{}", i))).unwrap();
+            storage
+                .save_conversation(&conv, Some(format!("session_{}", i)))
+                .unwrap();
         }
 
         // List sessions