Take the server's shards and dates when the scan completes, not after the sweep

The derived sync fired only after the metadata sweep, so a fresh device
re-derived every thumbnail it scrolled past, re-detected faces and re-read
every header for hours before adopting the shards and snapshot that held
all of it. It now fires as soon as the scan completes — the first moment
the rows the merges key on exist — and the sweep starts behind it. In
steady state that pass is one listing.

The catalog merge gains a fourth half: capture metadata (captured_at,
offset, camera, lens, ISO) for images still at metadata_state < 2, matched
by oc:fileid from a remote row at 2. A date is a fact about the file's
bytes, not local state, and the snapshot already carried it. The sweep's
per-chunk query then finds nothing left, and the timeline is whole on a
fresh device without a header fetch.

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

ui/dr-ui/src/derived_sync.rs

@@ -67,6 +67,9 @@ pub struct SyncReport {
     /// devices already had gains *no* collection, and reporting only the
     /// former left the sidebar showing no count beside a full collection.
     pub members_gained: usize,
+    /// Images dated from the remote's snapshot rather than by this device's
+    /// own sweep — what makes the timeline whole on a fresh device.
+    pub dates_gained: usize,
 
     // Face data is counted apart from thumbnails for the same reason keywords
     // are counted apart from collections: "adopted 4,812 faces" is a sentence
@@ -815,6 +818,7 @@ fn merge_downloaded(
     report.catalog_merged = true;
     report.collections_gained += merge.inserted + merge.updated;
     report.members_gained += merge.members_added;
+    report.dates_gained += merge.metadata_adopted;
     Ok(())
 }