chore: add GitHub community files and CI pipeline

- .github/workflows/ci.yml: TypeScript check + Vitest + build on every PR
- .github/dependabot.yml: weekly npm + Actions updates (minor/patch grouped)
- .github/ISSUE_TEMPLATE/: bug_report, feature_request, config (→ Discussions)
- .github/PULL_REQUEST_TEMPLATE.md: checklist for contributors
- SECURITY.md: responsible disclosure policy via GitHub Advisories
- CONTRIBUTING.md: dev setup, code style, commit conventions

.github/dependabot.yml

@@ -0,0 +1,42 @@
+version: 2
+
+updates:
+  # npm dependencies
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+      time: "09:00"
+      timezone: "Europe/Berlin"
+    open-pull-requests-limit: 5
+    labels:
+      - "dependencies"
+    groups:
+      # Batch minor/patch Next.js updates together
+      nextjs:
+        patterns:
+          - "next"
+          - "eslint-config-next"
+      # Batch React updates
+      react:
+        patterns:
+          - "react"
+          - "react-dom"
+          - "@types/react*"
+    ignore:
+      # Don't auto-update major versions (review manually)
+      - dependency-name: "*"
+        update-types: ["version-update:semver-major"]
+
+  # GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+      time: "09:00"
+      timezone: "Europe/Berlin"
+    labels:
+      - "dependencies"
+      - "github-actions"