Replace CAS with Churros OAuth

.env.example

CONTRIBUTING.md

package.json

pnpm-lock.yaml

scripts/build.sh

src/app.html

src/lib/server/cas.ts

src/lib/server/oauth.ts

src/lib/stores.ts

src/routes/+layout.svelte

src/routes/account/+page.server.ts

src/routes/announcement/emails/+server.ts

src/routes/login/+page.server.ts

src/routes/login/+page.svelte

src/routes/login/callback/+page.svelte

src/routes/login/callback/done/+server.ts

src/routes/login/cas/+server.ts