fix: drag-to-transfer drop not triggering transfer dialog

s_dropTargetIdx was reset to -1 unconditionally each frame, including
the release frame. Since drop target detection runs in PASS 2 (after
the drop handler), the target was always -1 when checked. Only reset
while mouse button is held so the previous frame's value is preserved.

Also bump version to 1.2.0-rc1 and add release notes.

build.sh

@@ -20,7 +20,7 @@
 set -e
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
-VERSION="1.2.0"
+VERSION="1.2.0-rc1"
 
 # ── Colours ──────────────────────────────────────────────────────────────────
 RED='\033[0;31m'