Rename default branch from master to main

Context

The multei/web repository used master as the default branch.

Goal

Rename the default branch to main, matching current GitHub and open-source defaults.

Suggested scope

• Rename master → main on the remote (git branch -m / GitHub “Rename branch”, or equivalent).
• Set the repository default branch on GitHub to main.
• Update production/deploy branch (Netlify today / Vercel in #416) to build from main.
• Update branch protection rules, status checks, and environments that pointed at master. (no protection on master; nothing to migrate)
• Update references in code/docs/CI (README, workflows, badges, scripts, clone links).
• Communicate to the team / document local clone updates (git fetch + git remote set-head / tracking). (instructions in README)
• Remove remote master after validating everything points at main (or keep a temporary alias if needed).

Acceptance criteria

• GitHub default branch is main.
• Deploy/CI use main. (GitHub Actions on push already cover main; pending Netlify Production branch)
• No critical remaining references to master in docs/CI/deploy (except commit history).
• Existing clones have clear local migration instructions.

Notes

• Coordinate with #416 (Vercel) and #415 (Next.js) if production branch changes happen in the same window, to avoid conflicting cutovers.
• Remaining follow-up: point Netlify Production branch to main.