Fix bash trap variable scope bug in QR camera scanner

The cleanup_camera trap handler couldn't access local variables
(ffmpeg_pid, tmpdir) when triggered, causing "unbound variable"
errors with set -u. Fixed by using global paths with $$ suffix
that the trap can reference directly.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

setup

@@ -216,15 +216,18 @@ scan_qr_camera() {
         return 1
     fi
 
-    local tmpdir="/tmp/usync_qr_$$"
-    local result_file="$tmpdir/result"
-    local ffmpeg_pid=""
+    # Use fixed paths (with PID) that cleanup can access without local vars
+    USYNC_QR_TMPDIR="/tmp/usync_qr_$$"
+    USYNC_QR_PIDFILE="/tmp/usync_qr_pid_$$"
 
-    mkdir -p "$tmpdir"
+    mkdir -p "$USYNC_QR_TMPDIR"
 
     cleanup_camera() {
-        [[ -n "$ffmpeg_pid" ]] && kill "$ffmpeg_pid" 2>/dev/null || true
-        rm -rf "$tmpdir"
+        if [[ -f "/tmp/usync_qr_pid_$$" ]]; then
+            kill "$(cat "/tmp/usync_qr_pid_$$")" 2>/dev/null || true
+            rm -f "/tmp/usync_qr_pid_$$"
+        fi
+        rm -rf "/tmp/usync_qr_$$"
     }
     trap cleanup_camera RETURN EXIT
 
@@ -237,9 +240,9 @@ scan_qr_camera() {
     ffmpeg -f v4l2 -video_size 640x480 -i /dev/video0 \
         -vf "drawbox=x=iw/2-100:y=ih/2-100:w=200:h=200:color=green:t=3" \
         -f sdl2 -window_title "Scan QR Code" - \
-        -vf "fps=2" -update 1 -q:v 2 "$tmpdir/frame.jpg" \
+        -vf "fps=2" -update 1 -q:v 2 "$USYNC_QR_TMPDIR/frame.jpg" \
         -loglevel quiet 2>/dev/null &
-    ffmpeg_pid=$!
+    echo $! > "$USYNC_QR_PIDFILE"
 
     sleep 1  # Let camera warm up
 
@@ -248,13 +251,13 @@ scan_qr_camera() {
         sleep 0.5
 
         # Check if ffmpeg died
-        if ! kill -0 "$ffmpeg_pid" 2>/dev/null; then
+        if [[ ! -f "$USYNC_QR_PIDFILE" ]] || ! kill -0 "$(cat "$USYNC_QR_PIDFILE")" 2>/dev/null; then
             break
         fi
 
-        if [[ -f "$tmpdir/frame.jpg" ]]; then
+        if [[ -f "$USYNC_QR_TMPDIR/frame.jpg" ]]; then
             local result
-            result=$(zbarimg --raw -q "$tmpdir/frame.jpg" 2>/dev/null || true)
+            result=$(zbarimg --raw -q "$USYNC_QR_TMPDIR/frame.jpg" 2>/dev/null || true)
 
             if [[ -n "$result" ]]; then
                 result=$(echo "$result" | tr -d ' \n\r' | tr '[:lower:]' '[:upper:]')