refactor: remove lite variant, fix release workflow

- Remove all lite/full variant logic from frontend, API, shared constants,
  docs, and tests (single unified Docker image only)
- Replace single QEMU multi-arch Docker build with per-architecture native
  builds (amd64 + arm64) and manifest merge to fix disk space exhaustion
- Add disk cleanup step and per-platform build cache scopes
- Switch release trigger from push to workflow_dispatch
- Add GitHub issue templates and PR template

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,8 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Documentation
+    url: https://stirling-image.github.io/stirling-image/
+    about: Check the docs for setup guides, configuration, and API reference.
+  - name: Security vulnerability
+    url: https://github.com/stirling-image/stirling-image/security/advisories/new
+    about: Report security issues privately through GitHub Security Advisories.