feat: bell moves beside identity; deprecate the nav-row placement

[ditto-m]

Summary

Moves the notification bell out of the nav list and beside identity, per the corrected wiki/concepts/design-system §6 (etamong-playground/planning#1185).

The v0.43 placement put the bell in the nav list, where it read as a destination sitting next to Dashboard and Docs. It isn't one: our bell opens a popover of recent items and routes elsewhere — an ephemeral stream, not a triageable object with its own state. §6's test says that belongs next to identity, matching how GitHub, Vercel and Figma pair the bell with the avatar.

• <Sidebar footerAccessory> + NotificationBell variant="footer". Expanded, the bell is a trailing icon beside the UserMenu trigger; collapsed to the 64px rail it stacks directly above the avatar-degraded identity control, badge intact. Below 720px nothing changes — apps mount it on NavigationBar's trailing edge.
• variant="row" is deprecated, not removed — ~8 apps use it, so the migration stays orderly.
• The no-accessory path is byte-for-byte unchanged; footerAccessory is optional.

Adversarial review caught a real accessibility bug

The first implementation rendered the bell trailing via CSS flex-direction: row-reverse while keeping it first in the DOM (so the collapsed column would stack without a second ruleset). That desyncs Tab and screen-reader linear order — both follow DOM — from visual order: keyboard users reached the bell before the identity control, and screen readers announced the unread count before the account control, inverting the exact hierarchy this placement exists to express. WCAG 2.4.3 / 1.3.2.

Fixed by reordering the DOM in Sidebar from a footerCollapsed flag, gated on tabletMode === "rail" rather than the raw collapsed state (which spuriously reads collapsed at tablet width in drawer/full modes). Plain flex; no CSS reorder trick. A Tab-order regression test now guards it — the suite previously asserted geometry and DOM order separately and never cross-checked them, which is why this passed review invisibly.

Also added: a dev-only warning when a bell lands in footerAccessory without variant="footer", since the failure mode otherwise is a silently clipped popover with no console signal.

Deliberately not changed: the badge stays a pill at every width. §6's dot-degradation rule is scoped to SidebarItem.badge (nav rows); the bell's own badge is bounded to "99+" by the component and always fits the 40px trigger, so the condition never fires. Two reviewers reached opposite conclusions on this, so the docs now say why.

Verification

typecheck, build, vitest 136/136, showcase build, Playwright 18/18 — including the new Tab-order test and the rail-collapse visibility/badge/popover-open test. Screenshots reviewed at 1280×900 expanded, 1280×900 collapsed rail, and 390×844 in both themes.

Relates to etamong-playground/planning#1133