Implement chat and markdown views, enhance status bar, and add workflow panel

- Added `chat.rs` for rendering chat messages with timestamps and roles.
- Introduced `markdown.rs` for rendering markdown content with styling.
- Created `status.rs` to display the application status bar with session and message counts.
- Developed `workflow.rs` to show the current workflow status, including tool calls and active jobs.
- Established a `theme.rs` for centralized color management across the UI.
- Updated `mod.rs` to include new modules and manage rendering logic.

src/model/session_lock.rs

@@ -0,0 +1,43 @@
+use std::path::{Path, PathBuf};
+use std::fs;
+
+pub struct SessionLock {
+    path: PathBuf,
+    pid: u32,
+}
+
+impl SessionLock {
+    pub fn new(session_dir: &Path) -> Self {
+        SessionLock {
+            path: session_dir.join(".lock"),
+            pid: std::process::id(),
+        }
+    }
+
+    pub fn try_lock(&self) -> std::io::Result<bool> {
+        if self.path.exists() {
+            let content = fs::read_to_string(&self.path).unwrap_or_default();
+            if let Ok(pid) = content.trim().parse::<u32>() {
+                if self.is_alive(pid) {
+                    return Ok(false);
+                }
+            }
+        }
+        fs::write(&self.path, self.pid.to_string())?;
+        Ok(true)
+    }
+
+    pub fn unlock(&self) {
+        let _ = fs::remove_file(&self.path);
+    }
+
+    fn is_alive(&self, pid: u32) -> bool {
+        unsafe { libc::kill(pid as i32, 0) == 0 }
+    }
+}
+
+impl Drop for SessionLock {
+    fn drop(&mut self) {
+        let _ = fs::remove_file(&self.path);
+    }
+}