fix(backoffice): stop sharing the position cookie with Smart Office

Both apps wrote a cookie named `current-position-id` but stored
different ids in it — freight the `employeePositionId`, Smart Office the
`position.id`. On a shared domain each login overwrote the other's desk
selection, and the loser silently fell back to the first position.

Freight now uses `freight-current-position-id` through a small helper
that reads the old name once, so a session live across the deploy keeps
its desk, and clears it on every write.

Also mounts the position switcher in the freight dashboard header. It
only existed under /performance-management, so on every other page a
two-desk user had no way to switch and was stuck on whatever
`useAuthUser` defaulted to. It now hides below two positions rather than
showing a one-option dropdown to the single-desk majority.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

apps/edr-freight-web/backoffice/src/shared/utils/positionCookie.ts

@@ -0,0 +1,39 @@
+import Cookies from "js-cookie";
+
+type CookieOptions = NonNullable<Parameters<typeof Cookies.set>[2]>;
+
+/**
+ * Freight's own active-position cookie.
+ *
+ * Smart Office is a separate app on the same IAM and it also writes a cookie
+ * named `current-position-id` — but it stores `position.id` where freight
+ * stores `employeePositionId`. The two are not interchangeable, so on a shared
+ * domain each app's login silently overwrote the other's desk selection and the
+ * loser fell back to `positions[0]`. Freight keeps its own cookie name so both
+ * can hold a selection at once.
+ */
+export const POSITION_COOKIE = "freight-current-position-id";
+
+/**
+ * The pre-rename, shared-with-Smart-Office name. Still read so a session that
+ * is live across the deploy keeps its desk, and cleared on every write so the
+ * colliding cookie does not linger.
+ */
+const LEGACY_POSITION_COOKIE = "current-position-id";
+
+/** The active `employeePositionId`, or undefined when no desk is selected. */
+export const getPositionCookie = (): string | undefined =>
+  Cookies.get(POSITION_COOKIE) ?? Cookies.get(LEGACY_POSITION_COOKIE);
+
+export const setPositionCookie = (
+  value: string,
+  options?: CookieOptions,
+): void => {
+  Cookies.set(POSITION_COOKIE, value, options);
+  Cookies.remove(LEGACY_POSITION_COOKIE);
+};
+
+export const clearPositionCookie = (): void => {
+  Cookies.remove(POSITION_COOKIE);
+  Cookies.remove(LEGACY_POSITION_COOKIE);
+};