chatgui: auto-clean build dir on generator change

tools/chatgui/build.sh

@@ -18,7 +18,15 @@ log "Source:  $SRC_DIR"
 log "Build:   $BUILD_DIR"
 log ""
 
-log "[1/4] Configuring CMake..."
+if [ -f "$BUILD_DIR/CMakeCache.txt" ]; then
+    OLD_GEN=$(grep 'CMAKE_GENERATOR:' "$BUILD_DIR/CMakeCache.txt" 2>/dev/null | cut -d= -f2)
+    if [ "$OLD_GEN" != "Ninja" ]; then
+        log "Generator changed ($OLD_GEN -> Ninja), cleaning build dir..."
+        rm -rf "$BUILD_DIR"
+    fi
+fi
+
+log "[1/4] Configuring CMake (generator: Ninja)..."
 cmake -S "$SRC_DIR" -B "$BUILD_DIR" -G Ninja 2>&1 | tee -a "$LOG_FILE"
 test ${PIPESTATUS[0]} -eq 0 || { log "CMake configure FAILED"; exit 1; }