React migration

[LachlanJW]

AI led migration to React/Vite migration. Some notes:

1. This is approx +6000,-5000 line change but adds a bunch of fixtures, lockfiles, and tests hence the crazy 22,000 lines added thing. I have enjoyed using the fixtures to test locally without pointing at dev, but happy to remove.
2. UI library: AI liked Mantine, I started this migration before talking to Mike who has endorsed Radix (and I am definitely in, shouldnt be a hard refactor now). We can discuss, just wanted to see what you thought of the React main part first :)
3. TanStack Query for server state (maybe overkill but kinda cool, gives caching, refetch, etc)
4. Check out the README for new fun commands!
5. Personally I'd be tempted to drop playwright (huge and only really useful to test the chart rendering i think) but thought I'd leave it in to show you both.

I also reckon we move routes out of server.py and maybe even break them up by page, but not strictly needed.