ci: make npm cache step non-fatal (fixes recurring typecheck flake #828)

The TypeScript-typecheck job intermittently failed at 'Cache npm download
cache' (transient cache-backend hiccup), which skipped install + type check and
marked the run red — 3x during the issues+systems build, all on pushes the
cache step had no bearing on. continue-on-error: true degrades a cache failure
to 'install without cache' instead of failing the job.

Closes the rerun churn from task #828.

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

.forgejo/workflows/ci.yml

@@ -81,6 +81,11 @@ jobs:
 
       - name: Cache npm download cache
         uses: actions/cache@v4
+        # Non-fatal: a transient cache-backend hiccup must NOT fail the whole
+        # typecheck job (it was skipping install + type check and reporting red
+        # on backend-only pushes — see issue task #828). On cache miss/error the
+        # job just installs without the cache.
+        continue-on-error: true
         with:
           path: ~/.npm
           key: npm-cache-${{ hashFiles('frontend/package-lock.json') }}