Stop two stale ids reaching the API, and steer the drag test off free days

Post-merge verification of the six merged branches turned up two real
frontend defects and one stale test expectation.

Settings mounts before WorkspaceContext has populated, so PartnerInvitePanel
asked for invitations with an empty workspace id. The empty path segment
collapses in Vite's proxy, FastAPI matches the single-segment workspace route
with workspace_id="invitations", and the UUID parse 422s. Guarded the same way
useWorkspaceMembers already does.

PlanSyncProvider could sync a workspace id seeded from localStorage before
WorkspaceContext refreshed it — a workspace a reseed had since deleted. It
logged the resulting 404 and kept retrying it on every planVersion bump. A 404
now refreshes the workspace context instead, correcting the id rather than
narrating the failure forever.

The drag-create-delete expectation was simply out of date: vacation-typed saves
correctly exclude free days, and targetRange() only skipped months ending on a
Sunday, so a Monday-ending month put a Sunday in the range. It now skips any
free day, computed independently of the calendar data the backend itself reads.

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

frontend/e2e/support/dates.ts

@@ -60,6 +60,66 @@ export function whitSunday(easter: Date): Date {
   return addDays(easter, 49)
 }
 
+/** Midsummer's Day (Midsommardagen) — the Saturday falling between June 20
+ * and June 26 inclusive, per Swedish law. */
+export function midsummerDay(year: number): Date {
+  for (let day = 20; day <= 26; day++) {
+    const candidate = new Date(Date.UTC(year, 5, day)) // June = month 5
+    if (candidate.getUTCDay() === 6) return candidate
+  }
+  /* istanbul ignore next -- a Saturday always falls in a 7-day window */
+  throw new Error(`no Saturday found in June 20-26 ${year} — impossible`)
+}
+
+/**
+ * True for a Saturday, a Sunday, or a Swedish public/"de facto" free day —
+ * i.e. exactly the set of dates the planner treats as free (see
+ * `is_free_day`/`_FREE_CATEGORIES` in the backend's `app/utils/holidays.py`
+ * and `app/services/allocation_service.py`; `PlanningService.
+ * _reject_illegal_free_day` rejects a `vacation`-typed pin on any of them).
+ * Computed purely from date math, like `easterSunday` above — deliberately
+ * independent of the backend's `swedish_calendar.json` cache, so a test
+ * that uses this to pick "a range with no free days in it" isn't
+ * validating itself against the very data the system under test reads.
+ * Not meant to be exhaustive for every calendar nuance (klämdagar are
+ * intentionally excluded — they're workable, not free) — only for the
+ * free days a test needs to steer clear of.
+ */
+export function isSwedishFreeDay(date: Date): boolean {
+  const day = date.getUTCDay()
+  if (day === 0 || day === 6) return true // Sunday or Saturday
+
+  const year = date.getUTCFullYear()
+  const iso = toIsoDate(date)
+
+  const fixed = new Set([
+    `${year}-01-01`, // Nyårsdagen
+    `${year}-01-06`, // Trettondedag jul
+    `${year}-05-01`, // Första maj
+    `${year}-06-06`, // Sveriges nationaldag
+    `${year}-12-24`, // Julafton (de facto)
+    `${year}-12-25`, // Juldagen
+    `${year}-12-26`, // Annandag jul
+    `${year}-12-31`, // Nyårsafton (de facto)
+  ])
+  if (fixed.has(iso)) return true
+
+  const easter = easterSunday(year)
+  const midsummer = midsummerDay(year)
+  const moveable = [
+    addDays(easter, -2), // Långfredagen
+    addDays(easter, -1), // Påskafton (de facto)
+    easter, // Påskdagen
+    addDays(easter, 1), // Annandag påsk
+    addDays(easter, 39), // Kristi himmelsfärdsdag
+    addDays(easter, 48), // Pingstafton (de facto)
+    addDays(easter, 49), // Pingstdagen
+    addDays(midsummer, -1), // Midsommarafton (de facto) — midsummer day itself is always a Saturday, already covered above
+  ].map(toIsoDate)
+
+  return moveable.includes(iso)
+}
+
 /** Monday of the ISO week containing `date` (Mon=0 .. Sun=6 offset). */
 export function mondayOfWeek(date: Date): Date {
   const isoWeekday = (date.getUTCDay() + 6) % 7 // 0 = Monday