Bump vitest testTimeout to 15s to absorb parallel-run CPU contention

Previously the 5s default starved several tests during full-suite
parallel runs — most reliably OnlineBoardSearchPage.error's timeout
test and the eslint boundary tests that fork a lint process per check.
Each of those passes in isolation in ≤3s; the 3x headroom keeps them
stable without masking genuine hangs. Individual tests can still
override via the third arg to it().

vitest.config.ts

@@ -12,6 +12,13 @@ export default defineConfig({
     environment: "node",
     globals: true,
     passWithNoTests: true,
+    // 15s (up from the 5s default). Full-suite parallel runs occasionally
+    // starve single tests on slower hosts — most notably the CPU-heavy
+    // eslint rule tests and React Testing Library mounts. Keeps per-test
+    // timeouts long enough to absorb contention while still catching
+    // genuine hangs. Individual tests can still override via `it(..., fn, ms)`.
+    testTimeout: 15000,
+    hookTimeout: 15000,
     include: ["src/**/*.test.ts", "src/**/*.test.tsx", "tests/**/*.test.ts", "tests/**/*.test.tsx"],
     coverage: {
       provider: "v8",