feat: build lightrag webui frontend via bun in shellHook

Adds pkgs.bun to lightrag devShell and builds the React frontend
on first shell entry if lightrag/api/webui doesn't exist yet.

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

flake.nix

@@ -45,7 +45,7 @@
       devShells.${system} = {
 
         lightrag = pkgs.mkShell {
-          packages = [ pkgs.uv pkgs.python312 pkgs.curl ];
+          packages = [ pkgs.uv pkgs.python312 pkgs.curl pkgs.bun ];
 
           shellHook = ''
             RAGS_ROOT="$PWD"
@@ -59,6 +59,14 @@
             uv sync --extra api --extra offline-llm --quiet
             source .venv/bin/activate
 
+            if [ ! -d lightrag/api/webui ]; then
+              echo "Building frontend (first time, ~1min)..."
+              cd lightrag_webui
+              bun install --frozen-lockfile --silent
+              bun run build:bun
+              cd ..
+            fi
+
             if [ -f "$RAGS_ROOT/.env.lightrag" ]; then
               set -a; source "$RAGS_ROOT/.env.lightrag"; set +a
             fi