chore: add GitHub automation and fix rand vulnerability

GitHub Actions:
- Add Dependabot for weekly dependency updates (cargo, github-actions)
- Add PR labeler workflow with file-based auto-labeling
- Add issue/PR templates (bug report, feature request, PR template)
- Fix release.yml artifact paths and labeler.yml config syntax
Security:
- Upgrade rand 0.8 → 0.10 to fix RUSTSEC-2026-0097 unsoundness
- Update screenshot.rs for rand 0.10 API (thread_rng → rng, gen_range → random_range)

.github/workflows/labeler.yml

@@ -0,0 +1,15 @@
+name: PR Labeler
+
+on:
+  pull_request:
+    types: [opened, synchronize]
+
+jobs:
+  label:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Label PR based on changed files
+        uses: actions/labeler@v5
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          configuration-path: .github/labeler.yml