feat: adopt @mozempk/ig-auth shared Instagram session

Prefer the shared, kept-alive IG session (cookies + captured Chrome User-Agent)
over the manually-dropped secrets/cookies.txt, falling back to it when the shared
store is not configured. Dockerfile/CI resolve the scoped package via a BuildKit
secret.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -27,6 +27,9 @@ jobs:
           target: tester
           push: false
           cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:buildcache
+          # read token for the @mozempk/ig-auth scoped registry (npm ci)
+          secrets: |
+            gitea_npm=${{ secrets.REGISTRY_TOKEN }}
 
       - name: Log in to Gitea Container Registry
         uses: docker/login-action@v3
@@ -44,3 +47,6 @@ jobs:
           cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:buildcache
           cache-to: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:buildcache,mode=max
           platforms: linux/amd64
+          # read token for the @mozempk/ig-auth scoped registry (npm ci)
+          secrets: |
+            gitea_npm=${{ secrets.REGISTRY_TOKEN }}