Refactor: Extract monospace_attrs() helper to eliminate code duplication

- Add monospace_attrs() function in main.rs to centralize monospace font attributes creation
- Replace duplicated Attrs::new().family(Family::Monospace) calls in:
  * line_number.rs: LineNumberCache::get() method
  * main.rs: App::init() font enumeration
  * tab.rs: EditorTab::new() constructor
- Remove unused imports (Attrs, Family) from line_number.rs
- Resolves TODO comments about code repetition across modules

This change improves maintainability by having a single source of truth
for monospace font configuration.

src/tab.rs

@@ -47,8 +47,7 @@ pub struct EditorTab {
 
 impl EditorTab {
     pub fn new(config: &Config) -> Self {
-        //TODO: do not repeat, used in App::init
-        let attrs = Attrs::new().family(cosmic_text::Family::Monospace);
+        let attrs = crate::monospace_attrs();
         let zoom_adj = Default::default();
         let mut buffer = Buffer::new_empty(config.metrics(zoom_adj));
         buffer.set_text(