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/settings.slint

@@ -126,6 +126,15 @@ export component SettingsPage inherits Rectangle {
     in property <bool> browse-loading: false;
     /// At the library root, so there is nowhere up to go.
     in property <bool> browse-at-root: true;
+    /// Whether the destination is one that can be walked.
+    ///
+    /// A boolean from Rust rather than a test on `target-selected`. The index
+    /// was hardcoded to 1, which was Remote's position while both targets were
+    /// offered — and the moment Android's list narrowed to Remote alone, that
+    /// index became 0 and the button vanished on the one platform where it is
+    /// the *only* way to set a destination. An index into a list whose length
+    /// varies is not a fact about the target.
+    in property <bool> browse-available: false;
 
     callback format-picked(int);
     callback quality-changed(int);
@@ -561,7 +570,7 @@ export component SettingsPage inherits Rectangle {
                         // exact spelling of a path three levels down is how a
                         // destination silently becomes a new folder at the
                         // root.
-                        if root.target-selected == 1 && !root.browse-open: HorizontalLayout {
+                        if root.browse-available && !root.browse-open: HorizontalLayout {
                             alignment: start;
                             Button {
                                 text: "Choose folder…";