* fix(ds): cross-viewport lock — consistent auth mode-switch (#2137)
The auth sign-in/sign-up mode switch rendered as a segmented pill toggle on
mobile (md:hidden) but a plain text link on desktop (hidden md:block) — the
audit's cross-viewport inconsistency. Lock to the segmented switch at all widths
and drop the now-redundant desktop text links.
Verified on /sessions/new at desktop width: the Sign in / Create account
segmented toggle now shows (was a text link).
Note: the switch still uses the bespoke bg-surface-inset track; migrating it to
DS::SegmentedControl (#2145) would also fix its dark-mode contrast — a follow-up
once that lands. The account-new icon glyph-vs-chip case the audit grouped here
did not reproduce (account_type + method_selector already use consistent
DS::FilledIcon / chip icons).
* fix(auth): keep the sign-in/up switch after a failed submit
A failed sign-in/up POST re-renders :new from the #create action, so the
switch (gated on action_name == "new") disappeared and the active tab was
derived from current_page?, which breaks on the POST URL. Render the switch on
both new and create, and derive the active tab from controller_name. Addresses
Codex review on #2149.
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