ci: pin GitHub Actions to commit SHAs (#260)

Pin every external `uses: owner/action@tag` in .github/workflows/ to a
full 40-char commit SHA, keeping the version tag as a trailing comment so
humans and Dependabot can still read the intended version.

SHA pinning is the GitHub-recommended hardening for third-party actions:
a moving tag can be repointed at malicious code, a commit SHA cannot.

dependabot.yml already declares the github-actions ecosystem (weekly),
so it will keep these pins current by bumping the SHA + comment together.
No dependabot change needed.

Local reusable-workflow ref (./.github/workflows/release_desktop.yml) is
left as-is; only external references are pinned.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: korbinian90

.github/workflows/build_pwa_preview.yml

@@ -79,23 +79,23 @@ jobs:
       # which is what reviewers see in the GitHub diff. Repository is implicit.
       - name: Checkout PR merge ref (pull_request)
         if: github.event_name == 'pull_request'
-        uses: actions/checkout@v6
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6
         with:
           persist-credentials: false
 
       # workflow_dispatch: explicit head OID resolved from the PR. For fork PRs
       # this checks out the fork repo at the PR's head commit.
       - name: Checkout PR head (workflow_dispatch)
         if: github.event_name == 'workflow_dispatch'
-        uses: actions/checkout@v6
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6
         with:
           repository: ${{ steps.pr.outputs.repo }}
           ref: ${{ steps.pr.outputs.ref }}
           persist-credentials: false
 
-      - uses: pnpm/action-setup@v6
+      - uses: pnpm/action-setup@0ebf47130e4866e96fce0953f49152a61190b271 # v6
 
-      - uses: actions/setup-node@v6
+      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6
         with:
           node-version: '22'
           cache: 'pnpm'
@@ -134,7 +134,7 @@ jobs:
           EOF
 
       - name: Upload build artifact
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
         with:
           name: pwa-preview
           path: apps/pwa/build