feat(collections): import a file into a list that already exists

The import dialog always made a new list. It now asks where the places go:
a new list, as before and still the default, or a list that is already there.
The picker offers the lists the person may edit, which is their own plus a
shared one where they are an editor or an admin, and starts on the list they
have open.

Adding to a list only ever adds. A place the list already holds is left
exactly as it is, with its status, notes, rating and labels, and counted as
already there in what the import reports; it is recognised the way a single
save recognises one, by provider id, then name, then position. The list keeps
its own name, colour, icon and description whatever the file says, a label
the file brings is created only when the list has none of that name, and new
places are appended after the ones already there so a manual order survives.

The route is POST /api/addons/collections/:id/import, 200 because it adds to
a list rather than making one, and both imports now write their places through
the same helper.

client/src/api/collections.ts

@@ -12,6 +12,7 @@ import type {
   CollectionImportablesResponse,
   CollectionFile,
   CollectionImportRequest,
+  CollectionImportIntoRequest,
   CollectionImportResult,
   CollectionGpxExport,
   CollectionGpxReadRequest,
@@ -65,6 +66,9 @@ export const collectionsApi = {
     ax.get(`${base}/${id}/export`).then((r: AxiosResponse) => r.data),
   importFile: (body: CollectionImportRequest): Promise<CollectionImportResult> =>
     ax.post(`${base}/import`, body satisfies CollectionImportRequest).then((r: AxiosResponse) => r.data),
+  /** The same file into a list that already exists: it only ever adds to it. */
+  importFileInto: (id: number, body: CollectionImportIntoRequest): Promise<CollectionImportResult> =>
+    ax.post(`${base}/${id}/import`, body satisfies CollectionImportIntoRequest).then((r: AxiosResponse) => r.data),
   // The same list as GPX (#2301). A GPX is read by the server into a list file,
   // which then goes through importFile above like any other.
   exportGpx: (id: number): Promise<CollectionGpxExport> =>