Workspace. It was Main Menu > Window > Workspace. Which was set to "3D" instead of "Essentials (Default)".
How incredibly annoying. There is zero indication anywhere in the workspace to tell you what workspace you're currently in. And the only indicator is buried as an option under a submenu item. That's a poor User Experience.
The tools selected are 3D Editing Tools.
Select the tools without the little checkerboard next to them to edit standard layers by holding down the Control Key (Mac) or right clicking on the tool in the Toolbar until the popup tools appear.
You can also merely tap the tool shortcut repeatedly. For the Paint Bucket, you'd hold the Shift key and then tap the g ...
You can try to use the online Photoshop clone Photopea.
Drag and drop the image into Photopea.
Use Filter > Blur > Gaussian Blur to smoothen the image a bit:
Use Filter > Other > Maximum to make the lines a little bit thinner:
Use Filter > Other > Minimum to make the lines a little bit wider again:
Use Filter > Noise > Dust & ...