everyday: Sign in as your everyday account, instead of a dead end #913

Follow-up to #901, from product-reviewer-2's review.

Once an admin has an everyday account, submitting the form again answered "You already have an everyday account." with no way on. That happens, for example, after fixing the org's teams following a failed join.

Now:

• That message names the account and points at the way: "You already have an everyday account, maria-me: sign in as it below."
• The admin's settings line about the pair gains a Sign in as maria-me button. It posts to the new POST /settings/everyday/switch (admin scope, same origin), which ends this session and lands on /signin with "Sign in as maria-me, your everyday account."
• Someone with no everyday account gets 404 there.

Test (test/everyday.test.ts): the second submit names the account and shows the button. The switch clears the session cookie, redirects to /signin, and the old session no longer opens /settings. A plain user gets 404.

Checked: tsc is clean. everyday, first-login, home-feed, account and the cross files: 290 pass.

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