fix: get all Playwright E2E tests loading and expand CI to run full suite (#6009)

## Fix Playwright E2E tests and expand CI to run full suite

### Problem

The full Playwright suite was broken in two ways:

1. **`ConvertE2E.spec.ts` crashed at import time** —
`conversionEndpointDiscovery.ts` imported a React hook at the top level,
which pulled in the entire component tree. That chain eventually
required `material-symbols-icons.json` (a generated file that didn't
exist), crashing module resolution before any tests ran.

2. **CI only ran cert validation tests** — both `build.yml` and
`nightly.yml` hardcoded `src/core/tests/certValidation` as the test
path, silently ignoring everything else.

### Changes

**`ConvertE2E.spec.ts` — complete rewrite**
The old tests were useless in practice: all 9 dynamic conversion tests
were permanently skipped unless a real Spring Boot backend was running
(they called a live `/api/v1/config/endpoints-enabled` endpoint at
module load time). Replaced with 4 focused tests that use `page.route()`
mocking — no backend required, same pattern as
`CertificateValidationE2E`.

New tests cover:
- Convert button absent before a format pair is selected
- Successful PDF→PNG conversion shows a download button (mocked API
response)
- API error surfaces as an error notification
- Convert button appears and is enabled after selecting valid formats

**`conversionEndpointDiscovery.ts` — deleted**
Only existed to support the old tests. The `useConversionEndpoints`
React hook it exported was never imported anywhere else.

**`ReviewToolStep.tsx`**
Added `data-testid="download-result-button"` to the download button —
required for the happy-path test assertion.

**CI workflows (`build.yml`, `nightly.yml`)**
- Added a `Generate icons` step before Playwright runs (`node
scripts/generate-icons.js`) — the icon JSON is generated by `npm run
dev` locally but skipped by `npm ci` in CI
- Removed the `src/core/tests/certValidation` path filter so the full
suite runs

frontend/src/core/tools/Convert.tsx

@@ -37,7 +37,7 @@ const Convert = ({ onPreviewFile, onComplete, onError }: BaseToolProps) => {
   };
 
   const hasFiles = selectedFiles.length > 0;
-  const hasResults = convertOperation.files.length > 0 || convertOperation.downloadUrl !== null;
+  const hasResults = convertOperation.files.length > 0 || convertOperation.downloadUrl !== null || !!convertOperation.errorMessage;
   const settingsCollapsed = hasResults;
 
   // When operation completes, flag the next selection change to skip reset