fix(ci): correct Docker action versions to latest valid releases

docker/login-action@v4 and docker/setup-buildx-action@v4 don't exist,
causing "Build and Push Docker Images" job to fail on every CI run.
Reverts to @V3 (latest major). Also aligns build-push-action to @v6
across all workflows for consistency.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: remyluslosius

.github/workflows/ci.yml

@@ -621,10 +621,10 @@ jobs:
         uses: actions/checkout@v5
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v4
+        uses: docker/setup-buildx-action@v3
 
       - name: Log in to GitHub Container Registry
-        uses: docker/login-action@v4
+        uses: docker/login-action@v3
         with:
           registry: ghcr.io
           username: ${{ github.actor }}

.github/workflows/deploy.yml

@@ -182,7 +182,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Build and push backend image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           file: ./docker/Dockerfile.backend
@@ -194,7 +194,7 @@ jobs:
           cache-to: type=gha,mode=max
 
       - name: Build and push frontend image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           file: ./docker/Dockerfile.frontend

.github/workflows/release.yml

@@ -101,7 +101,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Build and push backend image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           file: ./docker/Dockerfile.backend
@@ -114,7 +114,7 @@ jobs:
           cache-to: type=gha,mode=max
 
       - name: Build and push frontend image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           file: ./docker/Dockerfile.frontend