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/settings_ui.rs

@@ -201,6 +201,9 @@ pub fn render(window: &AppWindow, controller: &SettingsController) {
     window.set_settings_target_labels(labels(targets.iter().map(|t| t.label())));
     window.set_settings_target_selected(index_of(targets, &s.export.target));
     window.set_settings_destination(s.export.active_destination().into());
+    // Derived from the target itself, never from its position in a list whose
+    // length differs by platform.
+    window.set_settings_browse_available(s.export.target.is_remote());
     // The field means different things either side of the choice, and a
     // placeholder saying which is cheaper than a paragraph under it.
     // The placeholder names what an empty field *means*, which differs by