Add responsive app shell, routes and navigation

Replace the catch-all placeholder with an AppShell that renders one
primary nav per breakpoint (chosen with matchMedia via useBreakpoint):
a dark pill bottom bar below 768 px, a 96 px rail with the "M" mark at
768-1199 px, and a 208 px sidebar with the MealQuest wordmark and the
"Our kitchen / Shared household" card at 1200 px and up. Nav order is
Home, Plan, Discover, Quick, Settings; NavLink marks the active item
aria-current="page", with nested Plan and Settings paths keeping their
parent current. The mobile scroll area reserves 100 px plus the safe
area beneath its content.

Register every handoff route plus /settings/household,
/settings/regulars and /settings/calendar, each with a placeholder
heading. Remove the default body margin so the shell fills the viewport.

Tests force each breakpoint with a matchMedia mock and assert the
routes, nav order, each layout's structure, aria-current and the
mobile bottom padding from the stylesheet.

Maestro task: 42a4695a-6bd0-49a1-925e-387858dbee48

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/pages/PlaceholderPage.tsx

@@ -0,0 +1,9 @@
+export type PlaceholderPageProps = {
+  // The route's screen name, shown as the page heading.
+  title: string
+}
+
+// Stands in for a screen until its feature is built.
+export function PlaceholderPage({ title }: PlaceholderPageProps) {
+  return <h1>{title}</h1>
+}