chore: add pnpm workspace with dev:frontend / dev:backend scripts

- Root package.json with dev:frontend, dev:backend, dev (parallel), build scripts
- pnpm-workspace.yaml linking frontend/ subpackage
- pnpm-lock.yaml for reproducible installs
- .gitignore: exclude root node_modules/

package.json

@@ -0,0 +1,18 @@
+{
+  "name": "sub-store",
+  "private": true,
+  "version": "1.0.0",
+  "description": "Sub-Store — subscription aggregation and proxy management",
+  "type": "module",
+  "scripts": {
+    "dev:frontend": "pnpm --filter sub-store-frontend dev",
+    "dev:backend": "go run . serve -c config/config.yaml",
+    "dev": "pnpm run /^dev:.*/",
+    "build:frontend": "pnpm --filter sub-store-frontend build",
+    "build:backend": "go build -o sub-store ."
+  },
+  "engines": {
+    "node": ">=18",
+    "pnpm": ">=9"
+  }
+}