Make the thumbnails while the originals are still in hand

An import read every byte of every original, uploaded them, and then left the
grid to fetch a preview range back out of each one over the network — for
files that had been on this machine minutes earlier.

The pixels are now made from the bytes already in memory, on the fast path
FR-CAT-3 names: locate the camera's own embedded JPEG, decode a few hundred
KB, downscale, orient. Never a demosaic. A file with no usable preview yields
none, which is not an error and simply leaves the grid to fetch one later.

The filing waits for the upload, and only the filing. The shard store is keyed
by `oc:fileid` (FR-NC-5) and that does not exist until the server has the
file — so the thumbnail is made from the local copy and held until an id can
be attached to it. One listing per folder supplies every id at once, rather
than a PROPFIND per photograph over a link that may be mobile data.

Best-effort throughout: a thumbnail that cannot be filed costs the grid one
preview fetch later, and failing an import over it would be the wrong trade.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/src/import_ui.rs

@@ -323,6 +323,12 @@ where
         let context = context.clone();
         window.on_import_open(move || {
             let Some(w) = weak.upgrade() else { return };
+            // The button that reaches here is hidden where this is false, so
+            // arriving anyway means a key binding or a stale window state.
+            // Refused rather than shown empty.
+            if !dr_plat::imports_supported() {
+                return;
+            }
             // Re-read the settings file for the same reason the settings page
             // does: another instance may have written it since.
             *ctl.settings.settings.borrow_mut() = ctl.settings.store.load();