Remove non-existent labels from dependabot.yml

Dependabot was logging warnings on every PR because the configured
labels (frontend, backend, dependencies, etc.) don't exist as GitHub
labels in this repository. Removing them stops the noise.

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

.github/dependabot.yml

@@ -7,9 +7,6 @@ updates:
       interval: "weekly"
       day: "monday"
     open-pull-requests-limit: 5
-    labels:
-      - "dependencies"
-      - "frontend"
 
   # Backend npm dependencies
   - package-ecosystem: "npm"
@@ -18,9 +15,6 @@ updates:
       interval: "weekly"
       day: "monday"
     open-pull-requests-limit: 5
-    labels:
-      - "dependencies"
-      - "backend"
 
   # Docker dependencies (docker-compose.yml base images)
   - package-ecosystem: "docker"
@@ -28,18 +22,12 @@ updates:
     schedule:
       interval: "weekly"
       day: "monday"
-    labels:
-      - "dependencies"
-      - "docker"
 
   - package-ecosystem: "docker"
     directory: "/nginx"
     schedule:
       interval: "weekly"
       day: "monday"
-    labels:
-      - "dependencies"
-      - "docker"
 
   # GitHub Actions workflow dependencies
   - package-ecosystem: "github-actions"
@@ -47,6 +35,3 @@ updates:
     schedule:
       interval: "weekly"
       day: "monday"
-    labels:
-      - "dependencies"
-      - "github-actions"