[tui] Set tab title to file name.

Also update to use anyhow::Result in some places.

src/tui/app.rs

@@ -65,8 +65,16 @@ impl<'a> App<'a> {
     }
 
     fn draw_tabs(&self, area: Rect, buf: &mut Buffer) {
-        // TODO: Tabs should be opened from file explorer
-        Tabs::new(["file.md", "file.cpp"])
+        // Determine the tab title from the current file (or use a fallback).
+        let title = self
+            .editor
+            .file_path
+            .as_ref()
+            .and_then(|p| p.file_name())
+            .and_then(|s| s.to_str())
+            .unwrap_or("Untitled");
+
+        Tabs::new(vec![title])
             .divider(symbols::DOT)
             .block(
                 Block::default()