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/session.rs

@@ -231,7 +231,15 @@ impl SessionController {
         if conv.messages.len() == 1 {
             let first_msg = &conv.messages[0];
             let preview = first_msg.content.chars().take(50).collect::<String>();
-            return Ok(format!("{}{}", preview, if first_msg.content.len() > 50 { "..." } else { "" }));
+            return Ok(format!(
+                "{}{}",
+                preview,
+                if first_msg.content.len() > 50 {
+                    "..."
+                } else {
+                    ""
+                }
+            ));
         }
 
         // Build a summary prompt from the first few and last few messages
@@ -240,7 +248,8 @@ impl SessionController {
         // Add system message to guide the description
         summary_messages.push(crate::types::Message::system(
             "Summarize this conversation in 1-2 short sentences (max 100 characters). \
-             Focus on the main topic or question being discussed. Be concise and descriptive.".to_string()
+             Focus on the main topic or question being discussed. Be concise and descriptive."
+                .to_string(),
         ));
 
         // Include first message
@@ -283,7 +292,15 @@ impl SessionController {
                 if description.is_empty() {
                     let first_msg = &conv.messages[0];
                     let preview = first_msg.content.chars().take(50).collect::<String>();
-                    return Ok(format!("{}{}", preview, if first_msg.content.len() > 50 { "..." } else { "" }));
+                    return Ok(format!(
+                        "{}{}",
+                        preview,
+                        if first_msg.content.len() > 50 {
+                            "..."
+                        } else {
+                            ""
+                        }
+                    ));
                 }
 
                 // Truncate if too long
@@ -298,7 +315,15 @@ impl SessionController {
                 // Fallback to simple description if AI generation fails
                 let first_msg = &conv.messages[0];
                 let preview = first_msg.content.chars().take(50).collect::<String>();
-                Ok(format!("{}{}", preview, if first_msg.content.len() > 50 { "..." } else { "" }))
+                Ok(format!(
+                    "{}{}",
+                    preview,
+                    if first_msg.content.len() > 50 {
+                        "..."
+                    } else {
+                        ""
+                    }
+                ))
             }
         }
     }