refactor(web): replace floating-form store bus with idiomatic floating-ui

[tyler-dane]

Context

The Day view positioned its event form through a hand-rolled global state bus in useOpenAtCursor.ts — five createExternalStore singletons (open, nodeId, placement, strategy, reference) read via useSyncExternalStore and written imperatively from interaction handlers, plus a useFloatingAtCursor wrapper and a setTimeout(…, 10) / queueMicrotask to fight timing. All of it existed to feed one useFloating instance shared between the event form and the context menu (discriminated by a CursorItem enum).

Meanwhile the rest of the app — including the Week view (useEventForm) — already uses floating-ui the intended way: a local useFloating per component with the reference attached via refs.setReference or a virtual element at the cursor. The Day view was the lone outlier.

This PR converges both views onto the existing useEventForm hook and deletes the store bus.

What changed

• Open state is derived from Redux. A new status.isFormOpen flag on the draft slice (kept distinct from isDrafting/activity — drag-to-create makes a gridClick draft without opening the form) plus a selectIsEventFormOpen selector. Replaces openStore + nodeIdStore.
• The form anchors to the rendered draft element via refs.setReference, merged into the active draft card with useMergeRefs — exactly like the Week view's GridDraft. Removes the registry lookups, queueMicrotask, and setTimeout. Replaces referenceStore.
• useEventForm is the shared hook for both views, extended with optional floating/dismiss overrides. The Day view supplies its own positioning middleware (relocated to dayEventFormFloating.ts); the Week path is unchanged.
• The Day context menu gets its own local useFloating + virtual element at the cursor (matching GridContextMenuWrapper), removing the shared instance, the CursorItem discriminator, and the hidden anchor div.
• Deletes useOpenAtCursor.ts and useFloatingAtCursor.ts. The shared external-store.util is kept (other consumers remain). Net −350 lines.

Verification

• bun run type-check and biome lint: clean.
• bun test (events ducks, Day, Forms, Week views): 286 pass, 0 fail, including the updated DayInteractionCoordinator and useCloseEventForm suites.
• Manual, in the running app (temporary account → Day view):
  • Click an event → form opens anchored to it; outside-click dismisses and discards.
  • Right-click an event → context menu opens at the cursor, independent of the form.
  • Context-menu Edit → form opens.
  • No console errors across the flows.

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