feat(ui): build MegaMenu and rebuild Sidebar on Drawer

MegaMenu replaces a scaffold with a trigger and a wide panel of grouped link
columns. One level deep on purpose: a mega menu exists to show breadth flat so
everything is one click away, and nesting inside the panel buries content
behind hover-within-hover. Nav is the component for cascading submenus. The
panel is anchored so the runtime clamp keeps it inside the viewport.

Sidebar now composes Drawer for its off-canvas presentation instead of a
hand-rolled backdrop, inheriting the focus trap and scroll lock from one
place. Single items, labelled groups and branches nested to three levels, with
vertical roving focus.

Sidebar classes move to the BEM naming the rest of the library uses, which is
a breaking change; nesting via children still works, since that is what
shipped in 0.8.5.

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

docs/ui-visual-contract-0.8.json

@@ -61,11 +61,11 @@
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-    "packages/ui/components/Nav.wrn": "544483dbe7f2c3107c9c315ac0e7032c0989d8c7ad90e05a27ece12ac0652dfd",
+    "packages/ui/components/Nav.wrn": "7dc456b879e9248bde267b986cdcb138b8f3127d98b51e27bc9708129953f5e2",
     "packages/ui/components/Navbar.wrn": "e68f9d3643e500e43124e9c7a6d4c7f6722657377ea7313f3e3cf5093a81b360",
     "packages/ui/components/PageHeader.wrn": "0761235a4924eec09877be40b292d27b70db22d210be7d8e989493165c20df86",
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@@ -82,14 +82,14 @@
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     "packages/ui/components/StyledIcon.wrn": "4a4504e357dee9dd0418edbe85fc90b824ccdb3752123a2125da95b77bf0b336",
     "packages/ui/components/Switch.wrn": "ccb74599fab72b0d68b09a7f1f90b7732cb2f9cbed67a84219e897575ce30c28",
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