Fix language detection bit swap in T42 header parsing

Correctly map C12 and C13 control bits to fix misidentification of Swedish/Finnish (010) and German (001).
Also ensures Page model, Renderer, and UI properly propagate and display the detected language.

src/teletext/renderer.py

@@ -139,6 +139,12 @@ class TeletextCanvas(QWidget):
     
     def set_page(self, page: Page):
         self.page = page
+        # Set language from page header
+        if page:
+            self.subset_idx = page.language
+        else:
+            self.subset_idx = 0
+            
         self.cursor_x = 0
         self.cursor_y = 0
         self.redraw()