UI tests with Playwright

[blakerosenthal]

Added some tests for the UI with Playwright! Here's a couple decisions I made that could use another gut check or two:

• I chose to spin up an actual uvicorn server for the API rather than mock those endpoints. I also chose not to mock the UI endpoints for the sake of actually testing browser integration.
• I set different ports for the API and the UI so as to not conflict with other servers that might be running on the same machine.

Questions:

• What's happening with my auth setup? Setting a token either in the browser header or the panel cookies/headers doesn't seem to bypass the main auth page.
• Where should I look for better CSS locators? Playwright codegen offers some wacky identifiers. @pierrotsmnrd maybe you could point me to the right files for this?
• Will this work with GH Actions? Who knows; we'll find out!