Specify albums and pointing at folders, and describe them in the manual

FR-EXP-10 is the album: a named export destination beneath the
collections, whose folder holds only the exported files while the
catalog links each back to its original; how albums sync, why a device
folder does not, and why the tables are made on first use rather than
by a migration. FR-EXP-6 now says the destination is an album, never
inside the library, and that folders are chosen by pointing — the
portal or Windows dialogue, SAF's tree picker, the server browser —
each able to make a folder.

The manual's launch and export sections say the same in the words on
screen. Its pictures still show the 0.16.0 launch screen and export
settings; they are re-recorded with the rig, not edited by hand.

docs/manual/README.md

@@ -15,7 +15,10 @@ The photographs are the author's. None show a person.
 
 DarkRoom opens on a library: a folder on this machine, a folder a sync
 client keeps, or a Nextcloud account. A folder needs no password and uploads
-nothing.
+nothing. `Choose folder…` opens your desktop's own folder dialogue, which can
+make a new folder too; the folder used last stays on the screen with
+`Open folder` beside it. On a Nextcloud account the library folder is chosen
+in a browser of the server, whose `New folder` makes one there.
 
 ![The launch screen: a server field, a folder field, and which formats to scan for](media/launch.png)
 
@@ -300,9 +303,25 @@ as the first step in its history.
 ## Export
 
 `Export` in the develop header, or `Export N` from a selection. Format,
-size, colour space, sharpening, naming and where the file goes are in
-Settings, and apply to every export until changed. An export with no folder
-set is refused, and the header says so.
+size, colour space, sharpening and naming are in Settings, and apply to every
+export until changed. Where the file goes is an album.
+
+An album is a folder exports go to, listed under **Albums** in the sidebar,
+below the collections. Press `+` there, or `New album…` in the export sheet
+(`Ctrl+E`), name it, and choose its folder: on this device, in the system's
+folder dialogue (on the tablet, Android's folder picker), or on the server,
+in a browser that can make a folder as well as open one. Not inside the
+library — a JPEG exported there would come back from the next scan as a
+photograph of its own, and the browser says so rather than letting you.
+
+The folder holds only the exported files. The album remembers which
+photograph each came from, so selecting it in the sidebar shows the
+originals behind its JPEGs — edit one and export it again. Albums reach your
+other devices as collections do; a folder on this device does not, so an
+album made on the desktop asks the tablet for a folder of its own. Deleting
+an album leaves its files where they are.
+
+An export with no album chosen is refused, and the header says so.
 
 ![Export defaults in Settings](media/settings-export.png)