ci: add frontend build step before rust tests

The Tauri build requires the frontend/dist directory to exist, but the
workflow was trying to run Rust tests without building the frontend first.
Add a 'Build frontend' step between the frontend tests and Rust tests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: faiyaz26

.github/workflows/main-tests.yml

@@ -51,5 +51,8 @@ jobs:
       - name: Run frontend tests
         run: cd frontend && pnpm test
 
+      - name: Build frontend
+        run: cd frontend && pnpm build
+
       - name: Run Rust workspace tests
         run: cargo test --workspace