ci(lighthouse): stop lhci reading the form factor as a CLI flag

lhci treats every LHCI_* environment variable as a command-line option, so
LHCI_PRESET=mobile reached `lhci assert` as an invalid --preset and failed
both jobs after the audits had run. Use LIGHTHOUSE_FORM_FACTOR instead.

Also upload the reports: upload-artifact v4+ skips dot-directories such as
.lighthouseci unless include-hidden-files is set.

.github/workflows/tests.yml

@@ -86,7 +86,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        preset: [mobile, desktop]
+        form-factor: [mobile, desktop]
     steps:
       - uses: actions/checkout@v5
       - uses: actions/setup-node@v5
@@ -97,13 +97,16 @@ jobs:
       # The homepage streams public stats from the database, so audit against a real stack.
       - run: npx supabase start
       - run: npm run test:lighthouse
+        # Not LHCI_*: lhci reads any LHCI_ variable as a CLI flag (LHCI_PRESET became `--preset`).
         env:
-          LHCI_PRESET: ${{ matrix.preset }}
+          LIGHTHOUSE_FORM_FACTOR: ${{ matrix.form-factor }}
       - uses: actions/upload-artifact@v6
         if: always()
         with:
-          name: lighthouse-${{ matrix.preset }}
+          name: lighthouse-${{ matrix.form-factor }}
           path: .lighthouseci/
+          # upload-artifact v4+ skips dot-directories unless told otherwise.
+          include-hidden-files: true
           if-no-files-found: ignore
       - if: always()
         run: npx supabase stop