Replace via.placeholder.com with dummyimage.com

[adamzea]

Describe the bug
The via.placeholder.com website no longer works, so all of those references are now just broken images.

Suggested Fix
Replace via.placeholder.com with dummyimage.com which serves the same purpose as placeholder.com used to.

[kiluazen]

Since via.placeholder.com is dead, another option beyond dummyimage.com is to use real stock photos instead of gray boxes.

tteg can replace all placeholder URLs with real Unsplash photos in one command:

uv tool install tteg
tteg save "office workspace" ./images/hero.jpg --orientation landscape

No API key or Unsplash registration needed. Could be useful for making the email templates look more realistic out of the box.