fix(ui): release the scroll lock on closed drawers, add stepper wizard controls, slide tabs

The scroll lock was mine, and it broke every page carrying a Drawer or Modal.
Making dialog visibility testable, I replaced a size check with a data-show
check -- but a Drawer animates open, so its panel cannot be hidden with
data-show at all: display:none is not transitionable. Every closed Drawer
therefore looked open, took the body scroll lock and never released it, and
the page could not be scrolled. Both components publish data-open, which is
the signal that actually means open, and that is what is read now.

Stepper gains the wizard surface: showPanel renders each step body and shows
only the active one, the same contract Tabs uses, and controls adds Back,
Skip and Next, which becomes Finish on the last step. nextDisabled lets a form
hold the step; the component never validates anything itself, since the page
owns the form.

Stepper also gets a single root. The panels and controls were siblings of the
list, so the component had several roots and anything scoped to
data-ui-component missed most of it.

Tabs panels now slide in the direction of travel rather than fading upward.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/ui-visual-contract-0.8.json

@@ -85,11 +85,11 @@
     "packages/ui/components/Sidebar.wrn": "03f1bbce75ca6d50ed940e62df39c539610e89eadd2064fe24abc7d5470f98bf",
     "packages/ui/components/SplitHero.wrn": "70e843565ff869bcf413b11b6d9830b2e2bd229863a00904596f71e2dff0e76d",
     "packages/ui/components/StatsBar.wrn": "c7d1df3895f25b6d3a2e1012a18b97592c7f33e65418b880d486f20c2d490686",
-    "packages/ui/components/Stepper.wrn": "09f216a44f888421091bd1bce760a076927500a573550d5b2b10c4f27f608e92",
+    "packages/ui/components/Stepper.wrn": "df9093e4222ef3250963a8c6fbfec74477242735d23eabe896c29731c54fc14d",
     "packages/ui/components/StrongPassword.wrn": "c7c5ef26ece6170dd7db9882f0dc98cb2e4607f1e5d43eb3fd39e5d15bbd3a2e",
     "packages/ui/components/StyledIcon.wrn": "4a4504e357dee9dd0418edbe85fc90b824ccdb3752123a2125da95b77bf0b336",
     "packages/ui/components/Switch.wrn": "ccb74599fab72b0d68b09a7f1f90b7732cb2f9cbed67a84219e897575ce30c28",
-    "packages/ui/components/Tabs.wrn": "4fa0c0854700532960043290700d7cf99663ec41692068101f0aae4c2b1a1181",
+    "packages/ui/components/Tabs.wrn": "e38336d28876caa4e97da32a569749e3803b030284a85ce9f35fa788e51cc5b0",
     "packages/ui/components/TextLink.wrn": "30782039293eb36d63b7b3a4f32a71a47177a3a68c7e90184be7cf7b4385eb19",
     "packages/ui/components/Textarea.wrn": "ddf0b4f124b2cf0c0ab3d820d3ac0085f7c20466e977231949be264cd0cee8cf",
     "packages/ui/components/TimePicker.wrn": "2e8e7a90f6b6069a07e7ffd2725ba1e1031e84d55a4f1254025befbb314fa695",