Match thumbnail aspect ratio to singular view banner

[dd32]

Summary

• Changes the wccentral-thumbnail-large image size from 926x160 (5.79:1 ratio) to 926x418 (2.22:1 ratio) to match the wccentral-thumbnail-small thumbnail size (82x37, 2.22:1 ratio).
• This ensures the banner image on the singular WordCamp view uses the same aspect ratio as the schedule listing thumbnail, so uploaded images are not cropped differently between the two views.
• Existing images will need to be regenerated (via a thumbnails regeneration) for the new size to take effect.

Context

The schedule listing at central.wordcamp.org/schedule/ uses wccentral-thumbnail-small (82x37) while the singular view uses wccentral-thumbnail-large (926x160). These had very different aspect ratios (2.22:1 vs 5.79:1), causing banner images to appear cropped on the sides when shown as thumbnails.

Fixes #1617

Test plan

• Verify the image size registration in functions.php uses the updated dimensions
• Upload a test featured image and confirm the generated wccentral-thumbnail-large crop matches the aspect ratio of wccentral-thumbnail-small
• Check the singular WordCamp view displays the banner image correctly at the new ratio
• Confirm the schedule listing thumbnails still display correctly

Generated with Claude Code

[dd32]

Closing - submitted to wrong repo. Correct PR is on dd32/wordcamp.org.