fix: replace raw day-ID range checks with position-based helper (issue #889 follow-up)

Commit 8e05ba7 fixed the accommodation date-range pickers, but the
post-save state filters in DayDetailPanel and several other consumers
still compared `day.id >= start_day_id && day.id <= end_day_id`. With
non-monotonic ID layouts (day_number 1-9 → IDs 17-25, day_number 10-16
→ IDs 1-7) this made the just-saved accommodation immediately invisible
— matching the regression reported in the last comment of #889.

Introduces `isDayInAccommodationRange` in `client/src/utils/dayOrder.ts`
which compares positional order (`day_number` with `indexOf` fallback)
rather than raw IDs. Falls back to the old numeric comparison when
endpoint days are absent from the loaded array (sparse test data or
partial loads) so existing tests are unaffected.

Fixed call sites:
- DayDetailPanel.tsx (initial load, post-create, post-delete, post-edit-save)
- DayPlanSidebar.tsx (daily badge renderer)
- SharedTripPage.tsx (public share view)
- TripPDF.tsx (PDF export filter + sort)

Also declares `day_number?: number` on the client `Day` type (already
returned by the server but previously untyped).

Adds regression tests FE-PLANNER-DAYDETAIL-060/061/062 covering the
edit-save, create-save, and initial-load paths with the reporter's exact
non-monotonic ID layout.

client/src/types.ts

@@ -31,6 +31,7 @@ export interface Trip {
 export interface Day {
   id: number
   trip_id: number
+  day_number?: number
   date: string
   title: string | null
   notes: string | null