ci: harden CI and supply-chain pipeline (#1169)

* ci: harden CI and supply-chain pipeline

* Add frontend Vitest step to ci.yml so the 241 existing frontend tests run on
  every PR (mirrors the backend build/test/lint/audit order).
* Pin Node 26 as a single source of truth: new .node-version, node-version-file
  on all setup-node calls, engines.node ">=26.0.0" in all three package.json
  files. Matches the Dockerfile's node:26-alpine.
* SHA-pin remaining mutable actions in the start-app composite
  (actions/setup-node v6, actions/cache v4.3.0).
* Pin Dockerfile supply-chain inputs: golang:1.26.3-alpine by sha256 digest in
  both builder stages; replace mutable-tag git clone with commit-SHA fetch for
  docker/cli (v29.4.1) and docker/compose (v5.1.3). LDFLAGS version strings
  and otel patch preserved unchanged.
* Ref-scope docker-publish concurrency so two different release tags cannot
  cancel each other; same-ref reruns still cancel as before.
* Harden CLA workflow: drop actions:write from permissions; tighten the
  issue_comment trigger to PRs only (github.event.issue.pull_request != null)
  matching the two documented CLA phrases. No PR code is checked out.
* Drop trivy-version: latest from both Trivy scans so the SHA-pinned
  aquasecurity/trivy-action governs the bundled binary version. The
  HIGH/CRITICAL gate, severity filter, and trivy.yaml (OpenVEX) are unchanged.
* Restructure Dependabot: add applies-to: security-updates groups for npm
  (root/backend/frontend), docker, and github-actions; switch github-actions
  to directories so the local composite action is monitored alongside the
  top-level workflows.
* Add a daily scheduled SARIF security scan (security-scan.yml): two parallel
  jobs scanning saelix/sencho:latest and a fresh main HEAD build, uploading to
  GitHub code scanning. Least-privilege (contents: read, security-events:
  write). Visibility only; existing PR-blocking and release-blocking Trivy
  gates are not weakened.

Validation: backend tsc clean; frontend tsc clean; frontend npm test 27 files
/ 241 tests pass; npm audit --audit-level=high passes at root, backend, and
frontend; docker buildx build --check passes with no warnings (all pinned
digests resolve from the registry).

* ci(frontend): set explicit jsdom URL so localStorage initializes in CI

jsdom does not instantiate window.localStorage / sessionStorage when the
document has the opaque about:blank origin. Five frontend test files that
call localStorage.clear() in beforeEach started failing once the new
frontend Vitest step in this PR began running them on Linux CI runners.

Configuring environmentOptions.jsdom.url with a real same-origin URL is
the documented Vitest 4.x workaround and is a config-only change. All 27
test files (241 tests) pass locally with the fix applied.

.github/workflows/ci.yml

@@ -35,7 +35,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6
         with:
-          node-version: '20'
+          node-version-file: '.node-version'
           cache: 'npm'
           cache-dependency-path: backend/package-lock.json
 
@@ -85,7 +85,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6
         with:
-          node-version: '20'
+          node-version-file: '.node-version'
           cache: 'npm'
           cache-dependency-path: frontend/package-lock.json
 
@@ -97,6 +97,10 @@ jobs:
         working-directory: ./frontend
         run: npm run build
 
+      - name: Unit Tests (Vitest)
+        working-directory: ./frontend
+        run: npm test
+
       - name: Lint (ESLint)
         working-directory: ./frontend
         run: npm run lint
@@ -153,7 +157,6 @@ jobs:
           exit-code: '1'
           severity: 'CRITICAL,HIGH'
           format: 'table'
-          trivy-version: 'latest'
           trivy-config: trivy.yaml
 
       # Mirror of the release-time pre-publish smoke gate in docker-publish.yml.