Stop the export folder forgetting itself, and let Android reach one

Three faults, compounding into an export that could not be made to work on a
tablet at all and a destination that appeared to reset on its own.

**Switching target destroyed the destination.** One field held both a
filesystem path and a remote folder, so changing the target had to clear it —
`/home/x/Exports` carried to the server would have offered to create a folder
called `home` at the library root. The consequence was that merely looking at
the other option threw away the destination already chosen, which reads,
correctly, as a setting that will not stick. There are two fields now. Each
target remembers where it was pointed and switching is free; `active_destination`
picks between them so no caller can reach for the wrong one.

**The library root read as "unset".** The picker opens at the root, so
confirming it where it opens stored an empty string — indistinguishable from
"ask each time" and looking exactly like the picker had done nothing. Empty
now means the library root for a server destination, which is a real folder
and the one the photographs are already in; it means "ask" only for a device
folder, where no path is worth assuming. The page labels it so.

**Android defaulted to a target it cannot use.** A device folder there means
the Storage Access Framework, which provides no filesystem path (ARCH §6.9)
and is not implemented — so the default target could never succeed however the
destination was filled in. The export button said "no export folder is set",
the settings page offered no way to choose one, and the only way out was to
guess that the other target was the working one. The device target is now
absent from `ExportTarget::available()` on Android and the default there is the
server, which needs no platform work at all. A settings file carrying an
unreachable target — copied from a desktop, say — is corrected on read rather
than left to fail at the last step.

Seven tests, each named for the fault it prevents returning. The compatibility
one matters most: a file written before `remote_destination` existed keeps its
device path and gains an empty remote one.

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

ui/dr-ui/src/lib.rs

@@ -351,7 +351,7 @@ fn export_now(
     // since the server cannot be reached from here and may not be reachable
     // at all — see `export::stage` for why two queued exports of one name
     // both survive regardless.
-    let target_dir = std::path::PathBuf::from(&stored.export.destination);
+    let target_dir = std::path::PathBuf::from(stored.export.active_destination());
     let taken = |name: &str| -> bool {
         match stored.export.target {
             dr_types::ExportTarget::Device => target_dir.join(name).exists(),
@@ -373,7 +373,7 @@ fn export_now(
     export::place(
         &encoded,
         stored.export.target,
-        &stored.export.destination,
+        stored.export.active_destination(),
         &outbox,
     )
 }