Speed up frontend typecheck and lint, and fail test_frontend.sh on any error

tsc -b re-checked every file on every run: the configs named a
tsBuildInfoFile but never switched incremental on, and with noEmit there
are no outputs for -b to compare against. Enabling incremental in
tsconfig.app.json and tsconfig.node.json makes later runs reuse the
cache (28 s -> 0.5 s when nothing changed).

eslint ran on one thread with no cache. The lint script now uses
--concurrency auto and --cache in node_modules/.cache/eslint/
(42 s -> 14 s on a first run, 1.3 s after that). The config isn't
type-aware, so the cache is safe.

test_frontend.sh runs typecheck and lint in parallel, then the tests,
and exits 1 if any step fails. Before, a type or lint error scrolled
past and the exit code only reflected the tests.

test_frontend.sh

@@ -1,7 +1,29 @@
 #!/bin/sh
+# Typecheck and lint run in parallel (both are read-only and independent),
+# then the tests. Exits non-zero if any step fails, so a type or lint error
+# can't scroll past unnoticed behind a green test run.
+
+cd frontend || exit 1
+
+npm run typecheck > /tmp/bambuddy-typecheck.$$ 2>&1 &
+typecheck_pid=$!
+npm run lint > /tmp/bambuddy-lint.$$ 2>&1 &
+lint_pid=$!
+
+wait $typecheck_pid
+typecheck_rc=$?
+wait $lint_pid
+lint_rc=$?
+
+cat /tmp/bambuddy-typecheck.$$ /tmp/bambuddy-lint.$$
+rm -f /tmp/bambuddy-typecheck.$$ /tmp/bambuddy-lint.$$
 
-cd frontend
-npm run typecheck
-npm run lint
 npm run test:run
+test_rc=$?
+
 cd ..
+
+if [ $typecheck_rc -ne 0 ] || [ $lint_rc -ne 0 ] || [ $test_rc -ne 0 ]; then
+    echo "FAILED: typecheck=$typecheck_rc lint=$lint_rc tests=$test_rc"
+    exit 1
+fi