Show the folder picker on the platform that needs it most, and upload at once

Two faults, both of my own making, reported from the tablet as "I cannot
select a location" and "it does not upload".

**The picker button was gated on `target-selected == 1`.** That index was
Remote's position while both targets were offered. Making the target list
platform-aware narrowed Android's to Remote alone, so Remote became index 0
and the button disappeared — on the one platform where the picker is the
*only* way to set a destination, since a device folder is not reachable there
at all. It is gated on a boolean derived from the target now. An index into a
list whose length varies is not a fact about the target, and writing it as one
is what made a correct change break the thing it was meant to fix.

**A queued export waited for a sync pass.** Staging first is deliberate — an
export is finished on disk the moment it is written, and offline is then just
a longer queue — but nothing drained the outbox until the next sync, so
"Queued for Exports" sat unchanged and read, fairly, as an upload that never
happened. A finished batch that wrote anything now drains immediately. The
sync-pass drain stays: the first makes an upload feel immediate, the second is
what eventually delivers the exports made in a tunnel.

Committed without the parallel session's in-flight collection work, which is
mid-save and does not compile; verified by stashing it and building this tree
alone. 281 dr-ui tests pass, clippy clean.

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

ui/dr-ui/src/export.rs

@@ -889,6 +889,11 @@ pub fn drain_batch(
     rx: Receiver<BatchMessage>,
     total: usize,
     reporting: Reporting,
+    // Run when the batch finishes having written something. A queued export
+    // is finished on disk but not where the user asked for it, and waiting
+    // for the next sync pass to notice reads — correctly — as an export that
+    // did not upload.
+    on_exported: impl Fn() + 'static,
 ) {
     let job = activity.begin(
         crate::activity::Kind::Export,
@@ -958,6 +963,9 @@ pub fn drain_batch(
                         job.finish(text.clone());
                     }
                     settle(&w, reporting, &text);
+                    if exported > 0 {
+                        on_exported();
+                    }
                     stop_timer(&held);
                     return;
                 }