CI for multi-platform builds

[Mearman]

This PR sets up automated builds and releases for all platforms. Push a tag, get a release without manual building.

What:

• Builds automatically for Linux, Windows, and macOS (Intel + Apple Silicon)
• Tests run on every push and PR
• Creates installers: AppImage for Linux, DMG for macOS, EXE for Windows
• macOS universal binary. One download works on all Macs
• Dependabot keeps dependencies up to date

Why:

• Users can just download and run without building from source
• Maintainers save time on releases
• Contributors get instant CI feedback

Testing:

• [x] CI runs properly
• [x] All platforms build successfully
• [x] Binary validation works
• [ ] Need to test installers on real machines
• [ ] Need to verify universal binary on both Mac types

[acuteaangle]

The description contains every bizarre ChatGPT-ism under the sun.

I really can’t tell what’s not an LLM output in this. At least a few things stand out as weird to me; however, I’m too sleep deprived for code review at the moment.

[Mearman]

hahah I LLMed the description because I was likewise in a sleep deprived state. Thanks for the quick reply though. I'll write a more concise description later. TLDR, I thought it would be handy to have CI for the builds.