Fix gum file browser returning relative paths

gum file returns paths relative to the start directory.
Prepend / to ensure absolute paths for backup targets.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/ui_targets.sh

@@ -152,6 +152,8 @@ ui_target_folder_picker() {
                 path=$(gum file --directory --header "Select folder to back up" \
                     --cursor.foreground "$_GUM_ACCENT" --height 15 /) || continue
                 [[ -z "$path" ]] && continue
+                # gum file returns path relative to start dir — make absolute
+                [[ "$path" != /* ]] && path="/$path"
                 # Avoid duplicates
                 local dup=false
                 for f in "${folders[@]}"; do