Extends PutMatrixAccountRequest with a mode field (token, the existing
behavior and default; or password). Password mode has swarm-controller
itself perform m.login.password against the caller-given homeserver
(mirrors hive-c0re's own /api/matrix-account-login for the hive-local
case) and stores the resulting token instead of a caller-supplied one
-- the password is used once, over this PUT, and never stored. Also
adds the 'main is reserved' guard hive-c0re's login form already has,
which swarm-controller had no equivalent of before this.
swarm-ui's link-matrix-account form gets a credential-mode toggle
wired to the same contract: token mode is unchanged, password mode
swaps the token field for user-id + password fields and makes
homeserver required (no hive-side fallback to resolve it against, per
PutMatrixAccountRequest::homeserver's own doc).
Per #4122.
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