Merge backfill into Sync now, replace Full backfill with a destructive Reset all

Sync now now runs Backfill (resumes from the watermark, so a widened history
horizon is picked up automatically) then IncrementalSync then detail-fill, in
one click. "Full backfill" is gone -- in its place, "Reset all" (styled as a
destructive action, gated by a confirm dialog) wipes every synced activity
and its laps/kind assignments and rewinds the backfill watermark, so the next
sync performs a genuinely fresh pull instead of trying to patch up existing
rows with newer schema fields.

frontend/src/App.css

@@ -137,6 +137,17 @@ button:disabled {
   cursor: default;
 }
 
+.button-danger {
+  border-color: #7f1d1d;
+  color: #f87171;
+}
+
+.button-danger:hover:not(:disabled) {
+  background: #7f1d1d;
+  border-color: #f87171;
+  color: #fff;
+}
+
 .filter-pills {
   display: flex;
   gap: 0.5rem;