Updates to CartDrawer

Theming (both skins now themeable like the Website):

• The Vue skin's --rc-* colour tokens bridge to roadie-core ambient tokens (var(--background-color-raised, …), var(--color-accent-9, …), etc.), so the drawer inherits the collection accent (via --accent-hue) and dark mode (roadie tokens swap under .dark) — matching the React skin, which already inherits via roadie utility classes.
• New useRoadieTheme(accentColor) Vue composable mirrors React's ThemeProvider: derives OKLCH hue/chroma from a hex and writes --accent-hue/--accent-chroma to :root, so a Vue app can apply a collection theme the same way the Website does. Reuses core's sync OKLCH helpers — no new dependency.

Visual parity (both skins):

• Mobile shell morph — full-bleed flush-bottom with top-only rounding on mobile, floating card from sm up (was always floating).
• Spacing aligned to the Website (outer/section/ticket gaps); React drops a redundant wrapper and gains an opt-in roundedDayHeaders prop.
• Vue footer now has the progress-driven lift shadow the React skin already had.
• Centered empty state (icon, heading, prose) on both skins.
• Vue motion (bounce/pop) gated behind prefers-reduced-motion.

The header/footer measurement setters ran on every re-render (Vue re-invokes template ref callbacks each render), tearing down and re-observing their ResizeObserver; each re-observe fired the callback, whose deferred write scheduled another render → re-observe → a cross-frame loop that pinned the main thread. It surfaced as a hard tab freeze with no console error (one render per tick never trips Vue's in-tick recursion guard), and was worst during a drag because dragHeight re-renders on every pointermove.

createHeightTracker.setElement is now idempotent — it (re)observes only when a genuinely new element arrives and no-ops on same-element / transient-null re-invocations (real teardown stays in onScopeDispose). The header/footer ref callbacks are also hoisted to stable identities. Adds a regression test asserting the observer is not re-attached across re-renders.