Wires 1284's per-session token into the real write form and its real POST /send; adds tests for 1284/AC1-AC4
against that real page and handler, beside this issue's own AC1-AC4.
Accounts bucket fixture (accountBucketValue/accountBucketFolder) copied from mesh-mail's own
modules/mesh-mail/cmd/mesh-mail/mirror.go AccountValue/AccountFolderValue (issue 1255); mesh-mail-web is its
own Go module, so this is a copy, not an import.
Library choice: golang.org/x/oauth2 for the code exchange, github.com/coreos/go-oidc/v3 for the ID token's
verification (issuer, audience, expiry, signature from the issuer's JWKS).