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.

wiki/Collections.md

@@ -105,7 +105,12 @@ Sharing works between people on the same TREK. To give a list to somebody who ru
 - **TREK list** downloads the open list as a `.trekcollection.json` file: the list's name, description and colour, its labels, and every place with its address, coordinates, notes, status, website, phone, category name and labels. This is the one to give to another TREK.
 - **GPX** downloads the places as waypoints in a `.gpx` file, for OsmAnd, Organic Maps, a Garmin, gpx.studio or any other app that reads GPX. Each waypoint carries the place's name, description and address, notes, website and category, which every such app shows. Labels, status, phone and the rest travel alongside in a TREK extension that other apps ignore, so a GPX made by TREK comes back into TREK complete. A place without coordinates cannot be a waypoint: it is left out, and TREK tells you how many were.
 
-**Import** sits next to **New list** in the lists rail, as the button with the upload arrow. Pick a TREK list file or a GPX file and TREK shows what is in it before anything happens: the name, how many places, how many labels. Rename it there if you like, then import. The file always becomes a **new list of your own**, so nothing you already have can be overwritten, and you land on the new list when it is done.
+**Import** sits next to **New list** in the lists rail, as the button with the upload arrow. Pick a TREK list file or a GPX file and TREK shows what is in it before anything happens: the name, how many places, how many labels. Then choose where the places go, and you land on that list when it is done.
+
+- **New list** is the default: the file becomes a list of your own, under a name you can change right there.
+- **Add to a list** puts the places into a list you already have. The list is picked from the ones you may edit, which is your own lists plus a shared one where you are an editor or an admin; the list you have open is preselected.
+
+Adding to a list only ever adds. A place the list already has is left exactly as it is, with its status, notes, rating and labels, and counted as already there when the import reports what it did. TREK recognises it the same way it does when you save a single place: by the provider it came from, then by name, then by position. The list keeps its own name, colour, icon and description, whatever the file says about the list it came from; a label the file brings that the list does not have is created, and one it already has keeps its name and colour.
 
 From a GPX, every waypoint becomes a place, and so does a route point somebody named. A track is a line rather than a place, so it is not imported, and the preview says how many track points were left behind. A waypoint's type (OsmAnd files its favourites into groups this way) becomes the place's category when your palette has one of that name. A waypoint without a name is called "Waypoint" and its number in the file; one without usable coordinates is skipped and counted.