Update README.md with a few gotchas

From the PR description

Hey @willchen96 really enjoyed playing with Mike OSS.

I tried installing it as if I were a lawyer rather than an engineer and ran into a handful of papercuts that I think are easy fixes:

README changes

• Add a git clone step (the existing setup jumps straight to npm install, which assumes the reader has already cloned)
• Promote prerequisites (Node, git, Supabase, R2, model provider) into their own section with links so lawyers know what tools they're signing up for
• Walk through the steps in order: clone → install → env files → run schema → start backend → start frontend → sign up → add per-user API key
• Document the per-user API key gotcha - ANTHROPIC_API_KEY in .env is a server-side fallback only; the model picker is gated on a per-user key (or at least seemed to be)
• Note that Supabase moved its API page recently - Project URL is now under Settings → Data API, and the env vars expect the "Legacy" anon / service_role keys (not the new sb_publishable_* / sb_secret_* format) - possibly want to update this to new format
• Add an env-var reference table with where to get each value
• Mark LibreOffice as optional (only needed for .doc / .docx upload) - figure people could still play around with out it
• Add a Troubleshooting section covering the three issues I hit as a first-time user: npm install peer-dep warnings vs hard ERESOLVE failures, Supabase confirmation emails (rate-limited / team-restricted by default on newer Supabase projects) and the per-user model API key gating