ci: publish a test coverage report on every run

Adds npm run test:coverage (node:test experimental coverage via tsx)
and a CI step so line/branch/function coverage is visible on every
push and PR. The first report: 92% lines / 59% branches / 80% functions
overall, with the dashboard view at 100% after the render regression
tests; remaining gaps are the 3x-ui HTTP client (needs a live panel)
and the GitHub releases proxy.

Author: Z User

.github/workflows/ci.yml

@@ -25,5 +25,8 @@ jobs:
       - name: Run test suite
         run: npm test
 
+      - name: Test coverage report
+        run: npm run test:coverage
+
       - name: Production bundle build
         run: npm run build

package.json

@@ -8,7 +8,8 @@
     "build": "esbuild server.ts --bundle --platform=node --format=cjs --packages=external --outfile=dist/server.cjs",
     "start": "node dist/server.cjs",
     "lint": "tsc --noEmit",
-    "test": "tsx --test test/*.test.ts"
+    "test": "tsx --test test/*.test.ts",
+    "test:coverage": "tsx --test --experimental-test-coverage test/*.test.ts"
   },
   "dependencies": {
     "@types/adm-zip": "^0.5.8",