Shared Sign Cert Validation (#5996)

## PR: Certificate Pre-Validation for Document Signing

### Problem

When a participant uploaded a certificate to sign a document, there was
no validation at submission time. If the certificate had the wrong
password, was expired, or was incompatible with the signing algorithm,
the error only surfaced during **finalization** — potentially days
later, after all other participants had signed. At that point the
session is stuck with no way to recover.

Additionally, `buildKeystore` in the finalization service only
recognised `"P12"` as a cert type, causing a `400 Invalid certificate
type: PKCS12` error when the **owner** signed using the standard
`PKCS12` identifier.

---

### What this PR does

#### Backend — Certificate pre-validation service

Adds `CertificateSubmissionValidator`, which validates a keystore before
it is stored by:
1. Loading the keystore with the provided password (catches wrong
password / corrupt file)
2. Checking the certificate's validity dates (catches expired and
not-yet-valid certs)
3. Test-signing a blank PDF using the same `PdfSigningService` code path
as finalization (catches algorithm incompatibilities)

This runs on both the participant submission endpoint
(`WorkflowParticipantController`) and the owner signing endpoint
(`SigningSessionController`), so both flows are protected.

#### Backend — Bug fix

`SigningFinalizationService.buildKeystore` now accepts `"PKCS12"` and
`"PFX"` as aliases for `"P12"`, consistent with how the validator
already handles them. This fixes a `400` error when the owner signed
using the `PKCS12` cert type.

#### Frontend — Real-time validation feedback

`ParticipantView` gains a debounced validation call (600ms) triggered
whenever the cert file or password changes. The UI shows:
- A spinner while validating
- Green "Certificate valid until [date] · [subject name]" on success
- Red error message on failure (wrong password, expired, not yet valid)
- The submit button is disabled while validation is in flight

#### Tests — Three layers

| Layer | File | Coverage |
|---|---|---|
| Service unit | `CertificateSubmissionValidatorTest` | 11 tests — valid
P12/JKS, wrong password, corrupt bytes, expired, not-yet-valid, signing
failure, cert type aliases |
| Controller unit | `WorkflowParticipantValidateCertificateTest` | 4
tests — valid cert, invalid cert, missing file, invalid token |
| Controller integration | `CertificateValidationIntegrationTest` | 6
tests — real `.p12`/`.jks` files through the full controller → validator
stack |
| Frontend E2E | `CertificateValidationE2E.spec.ts` | 7 Playwright tests
— all feedback states, button behaviour, SERVER type bypass |

#### CI

- **PR**: Playwright runs on chromium when frontend files change (~2-3
min)
- **Nightly / on-demand**: All three browsers (chromium, firefox,
webkit) at 2 AM UTC, also manually triggerable via `workflow_dispatch`

.github/workflows/build.yml

@@ -217,6 +217,37 @@ jobs:
           path: frontend/dist/
           retention-days: 3
 
+  playwright-e2e:
+    if: needs.files-changed.outputs.frontend == 'true'
+    needs: files-changed
+    runs-on: ubuntu-latest
+    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@58077d3c7e43986b6b15fba718e8ea69e387dfcc # v2.15.1
+        with:
+          egress-policy: audit
+      - name: Checkout repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - name: Set up Node.js
+        uses: actions/setup-node@53b83947a5a98c8d113130e565377fae1a50d02f # v6.3.0
+        with:
+          node-version: "22"
+          cache: "npm"
+          cache-dependency-path: frontend/package-lock.json
+      - name: Install frontend dependencies
+        run: cd frontend && npm ci
+      - name: Install Playwright (chromium only)
+        run: cd frontend && npx playwright install chromium --with-deps
+      - name: Run E2E tests (chromium)
+        run: cd frontend && npx playwright test src/core/tests/certValidation --project=chromium
+      - name: Upload Playwright report
+        if: always()
+        uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f # v7.0.0
+        with:
+          name: playwright-report-pr-${{ github.run_id }}
+          path: frontend/playwright-report/
+          retention-days: 7
+
   check-licence:
     if: needs.files-changed.outputs.build == 'true'
     needs: [files-changed, build]

.github/workflows/nightly.yml

@@ -0,0 +1,50 @@
+name: Nightly E2E Tests
+
+on:
+  schedule:
+    - cron: "0 2 * * *" # 2 AM UTC every night
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  playwright-all-browsers:
+    name: Playwright (chromium + firefox + webkit)
+    runs-on: ubuntu-latest
+    steps:
+      - name: Harden the runner (Audit all outbound calls)
+        uses: step-security/harden-runner@58077d3c7e43986b6b15fba718e8ea69e387dfcc # v2.15.1
+        with:
+          egress-policy: audit
+
+      - name: Checkout repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: Set up Node.js
+        uses: actions/setup-node@53b83947a5a98c8d113130e565377fae1a50d02f # v6.3.0
+        with:
+          node-version: "22"
+          cache: "npm"
+          cache-dependency-path: frontend/package-lock.json
+
+      - name: Install frontend dependencies
+        run: cd frontend && npm ci
+
+      - name: Install all Playwright browsers
+        run: cd frontend && npx playwright install --with-deps
+
+      - name: Run E2E tests (all browsers)
+        run: cd frontend && npx playwright test src/core/tests/certValidation
+
+      - name: Upload Playwright report
+        if: always()
+        uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f # v7.0.0
+        with:
+          name: playwright-nightly-${{ github.run_id }}
+          path: frontend/playwright-report/
+          retention-days: 14