Refactor 'unsaved changes' modal to use KModal component and update tests

[Kartikayy007]

Summary

Replaced Vuetify's MessageDialog component with Kolibri Design System's KModal for the unsaved changes dialog in the Collections modal.

• Replaced MessageDialog with KModal in ChannelSetModal.vue
• Updated modal props to use KModal's API (title, submitText, cancelText)
• Maintained all existing functionality and event handlers

Manual verification:

1. Logged in as [email protected] with password a
2. Navigated to Channels > Collections
3. Clicked "New collection" button
4. Added text to Collection name input
5. Clicked close button in the top bar
6. Verified unsaved changes modal appears with KModal styling
7. Tested both "Exit without saving" and "Save and close" buttons
8. All functionality works as expected

References

• Fixes [Remove Vuetify from Studio] 'Unsaved changes' modal in Channels - New collection #5299

Reviewer Guidance

Test these changes by:

1. Login as [email protected] with password a
2. Go to Channels > Collections
3. Click "New collection" button
4. Add text to Collection name input
5. Click the close button in the top bar
6. Verify the unsaved changes modal appears and functions correctly
7. Test both cancel and save actions

Areas to focus on:

• Modal properly blocks navigation when unsaved changes exist
• Both RTL and LTR layouts work correctly
• All 19 tests in channelSetModal.spec.js pass