Show update count badge on Updates tab in ViewSwitcherBar

Add updatable_count() database method and refresh_update_badge()
window method. The Updates tab now shows a numeric badge with the
number of available updates and highlights with needs_attention.

src/core/database.rs

@@ -1257,6 +1257,16 @@ impl Database {
         Ok(())
     }
 
+    pub fn updatable_count(&self) -> i64 {
+        self.conn
+            .query_row(
+                "SELECT COUNT(*) FROM appimages WHERE latest_version IS NOT NULL",
+                [],
+                |row| row.get(0),
+            )
+            .unwrap_or(0)
+    }
+
     pub fn get_appimages_with_updates(&self) -> SqlResult<Vec<AppImageRecord>> {
         let sql = format!(
             "SELECT {} FROM appimages WHERE latest_version IS NOT NULL