The auth panel was a fixed 710x820 (640x790 on narrower desktop
windows) regardless of which view was showing, but Register and the
Steam linking form need up to ~990px to lay out without scrolling -
so the inner ScrollContainer was always kicking in for those views.
- Tightened form spacing (_new_form separation 16->10, field height
68->56, panel_content separation 24->16, page margins 56->28) to
shrink how much height each view actually needs.
- Size the panel to the tallest view's real requirement (1000/1010),
clamped to whatever height the window actually has so a short window
shrinks the panel - and lets the ScrollContainer take over as a
fallback - instead of the panel overflowing off-screen.
Verified headlessly across 1920x1080, 1366x768, and a narrow
compact-desktop width: no scrollbar in any mode at the two common
resolutions, and the panel now stays within the window (falling back
to scrolling only for the two tallest forms on the 768px-tall case,
rather than clipping off both edges of the screen as a fixed height
would have).