ui: collapsed rail hides identity footer; mobile bell sheet trapped by NavigationBar glass

[ditto-m]

Summary

Two rail/mobile reachability bugs found while adopting v0.44 across the fleet — both the same underlying story: the bell and the account menu must actually be reachable in the collapsed rail and on mobile, not just present in the DOM.

• Collapsed rail hides the identity footer (planning#1150). .etu-sidebar-footer was display: none in the collapsed rail (both the pre-hydration and post-hydration CSS blocks). Since v0.43 moved the theme toggle into the UserMenu popover, that popover's only entry point was the footer — so the collapsed rail had no way to reach identity, sign-out, or the theme toggle. Fix: the canonical <UserMenu variant="full"> footer now degrades to an avatar-only control (same ~40px target as other rail items) instead of disappearing. Name/email/badges still hide. A non-UserMenu custom footer node still collapses to nothing — it can't be safely rewritten to fit 64px without knowing its markup; that rule is now documented in the <Sidebar> doc comment and the CSS comment.

• Mobile bell sheet trapped by NavigationBar's glass (planning#1151). NotificationBell's default "trigger" variant renders its mobile sheet without a portal. NavigationBar always applies backdrop-filter (.etu-glass), which — like transform/filter/perspective — establishes a containing block for position: fixed descendants. A bell mounted in <NavigationBar trailing> (the documented mobile placement) had its sheet anchored to the header instead of the viewport, confirmed by bounding-box measurement in etamong-playground/pages. Fix: the mobile sheet + backdrop now portal to document.body, reusing the same createPortal call the row variant already used.

  • Checked for the same trap elsewhere: the trigger variant's desktop popover is unaffected (position: absolute against the bell's own position: relative wrapper, which wins the containing-block search before reaching any glass ancestor) — left untouched. <MobileTabBar> (the other .etu-glass consumer) renders no position: fixed descendants of its own. <UserMenu>'s dropdown and the cmdk <CommandPalette> dialog were already unconditionally portaled.

Bumped to 0.45.0 (minor, additive/behavioral-fix only — no prop removals or renames). See CHANGELOG.md for the full "Behavioral notes" section.

Test plan

• corepack pnpm typecheck
• corepack pnpm build
• corepack pnpm test (113 unit tests)
• corepack pnpm showcase:build
• CI=1 corepack pnpm e2e (16 Playwright tests, including two new regressions: rail-collapse footer trigger + popover, and mobile sheet bounding-box anchored to the viewport bottom)
• Manual visual check (Playwright screenshots, light + dark) of the collapsed rail with the UserMenu popover open, and the mobile NavigationBar bell sheet open — both confirmed correctly positioned before shipping
• Showcase (ChromeSection, App.tsx) already dogfoods both scenarios: the live sidebar footer is <UserMenu variant="full">, and the mobile <NavigationBar trailing> mounts a <NotificationBell> below 720px

Closes etamong-playground/planning#1150
Closes etamong-playground/planning#1151