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/ui/app.slint

@@ -616,6 +616,7 @@ export component AppWindow inherits Window {
     in property <[string]> settings-browse-entries;
     in property <bool> settings-browse-loading: false;
     in property <bool> settings-browse-at-root: true;
+    in property <bool> settings-browse-available: false;
     in property <[string]> settings-target-labels;
     in property <int> settings-target-selected: 0;
     in property <string> settings-error: "";
@@ -810,6 +811,7 @@ in property <bool> panel-visible: true;
             browse-entries: root.settings-browse-entries;
             browse-loading: root.settings-browse-loading;
             browse-at-root: root.settings-browse-at-root;
+            browse-available: root.settings-browse-available;
             target-labels: root.settings-target-labels;
             target-selected: root.settings-target-selected;
             error: root.settings-error;