everyday: a failed join keeps the admin signed in, at the org's teams #901

Follow-ups to #878, from product-reviewer-2's review.

On a spawned forge, making the everyday account adds it to the client's org as a writer. When no write team can be found or made there:

• Before: the admin was signed in as the new account, which could see nothing.
• Now: the account is still made and linked, but the admin stays signed in and lands on the org's page at its Teams section (/<org>#teams). A danger flash there says the account could not join, and that adding it to a team that writes there fixes it.

Tests (test/everyday.test.ts):

• joinAsWriter answers false when both candidate team names are taken by read-only teams.
• On a spawned forge, that case leaves no session for the new account. It redirects to /stalls#teams, and the page shows the message.

The org page's Teams heading gains id="teams"; the studio test that checks that heading is updated.

Checked: tsc is clean. everyday, first-login, orgs, studio and the cross files pass.

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