feat: add Flash/Steady toggle button to sidebar

src/teletext/ui.py

@@ -301,6 +301,14 @@ class MainWindow(QMainWindow):
         btn_release.clicked.connect(lambda: self.insert_char(0x1F))
         right_layout.addWidget(btn_release)
         
+        # Flash / Steady Toggle (0x08 / 0x09)
+        self.btn_flash_toggle = QPushButton("Flash/Steady")
+        self.btn_flash_toggle.setFixedSize(100, 30)
+        self.btn_flash_toggle.setStyleSheet("background-color: #FFCC00; color: black; font-weight: bold; border: 1px solid #555; border-radius: 3px;")
+        self.btn_flash_toggle.setToolTip("Toggles between Start Flashing (0x08) and Steady (0x09) at cursor.")
+        self.btn_flash_toggle.clicked.connect(self.toggle_flashing_attr)
+        right_layout.addWidget(self.btn_flash_toggle)
+        
         right_layout.addSpacing(10)
 
         # Page Language Setting
@@ -926,6 +934,29 @@ class MainWindow(QMainWindow):
         self.canvas.move_cursor(1, 0)
         self.canvas.setFocus()
 
+    def toggle_flashing_attr(self):
+        self.push_undo_state()
+        val = self.canvas.get_byte_at(self.canvas.cursor_x, self.canvas.cursor_y)
+        if val == 0x08:
+             new_val = 0x09 # Toggle to Steady
+        elif val == 0x07: # Let's say if we are on white, maybe we toggle flash?
+             new_val = 0x08 # No, that's not intuitive.
+             # If not flashing, make it flash.
+             # If flashing, make it steady.
+        else:
+             new_val = 0x08 # Default to flash if something else
+        
+        if val == 0x08:
+            new_val = 0x09
+        elif val == 0x09:
+            new_val = 0x08
+        else:
+            new_val = 0x08 # Default
+            
+        self.canvas.set_byte_at_cursor(new_val)
+        # We DON'T move cursor here so user can toggle again
+        self.canvas.setFocus()
+
     def insert_color(self, base_code):
         code = base_code
         if self.chk_graphics.isChecked():