Merge remote-tracking branch 'origin/studio' into feat/11577-fs-scale

# Conflicts:
#	.github/workflows/studio.yml
#	desktop/frontend-next/src/styles/sidepanel.css

.github/workflows/studio.yml

@@ -164,6 +164,7 @@ jobs:
           node perf/narrow-steps.mjs
           node perf/job-names.mjs
           node perf/fs-scale.mjs
+          node perf/composer-feel.mjs
           node perf/side-panel.mjs
           node perf/card-shell.mjs
           node perf/line-cards.mjs
@@ -178,6 +179,70 @@ jobs:
           node perf/narrow-rail.mjs
           node perf/selfcheck.mjs branch-tooltip
 
+  # What a hosted page does over the relay, end to end: the production bundle,
+  # the real Studio host and kernel, a real Chromium; only the accounts API and
+  # the relay are stand-ins (tools/relayfake). Event-transport changes are
+  # judged here, because a unit test of one side cannot see the other.
+  relay-harness:
+    name: relay harness
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
+        with:
+          fetch-depth: 0
+
+      - name: Transport changed?
+        id: scope
+        env:
+          BASE_REF: ${{ github.base_ref }}
+        run: |
+          paths="desktop/frontend-next internal/frontend/serve internal/platform/remotecloud internal/platform/account tools/relayfake cmd/reasonix-studio-host"
+          if [ "$GITHUB_EVENT_NAME" = "pull_request" ] &&
+             git diff --quiet "origin/$BASE_REF...HEAD" -- $paths; then
+            echo "run=false" >> "$GITHUB_OUTPUT"
+          else
+            echo "run=true" >> "$GITHUB_OUTPUT"
+          fi
+
+      - if: steps.scope.outputs.run == 'true'
+        uses: actions/setup-go@b7ad1dad31e06c5925ef5d2fc7ad053ef454303e # v7
+        with:
+          go-version-file: go.mod
+          cache: true
+
+      - if: steps.scope.outputs.run == 'true'
+        uses: pnpm/action-setup@ea17c68df8912ef543352723c149a84f56e3d413 # v6.1.0
+        with:
+          version: 10
+          run_install: false
+
+      - if: steps.scope.outputs.run == 'true'
+        uses: actions/setup-node@949feb2413d6458794dcd2491c4babbbce0c15c1 # v7
+        with:
+          node-version: "24"
+          cache: pnpm
+          cache-dependency-path: desktop/frontend-next/pnpm-lock.yaml
+
+      - if: steps.scope.outputs.run == 'true'
+        name: Build the host, the stand-ins and the page
+        run: |
+          go build -o desktop/frontend-next/relay/bin/studio-host ./cmd/reasonix-studio-host
+          go build -o desktop/frontend-next/relay/bin/relayfake ./tools/relayfake
+          cd desktop/frontend-next
+          pnpm install --frozen-lockfile
+          pnpm exec vite build --outDir dist-relay --emptyOutDir
+          pnpm exec playwright install --with-deps chromium
+
+      - if: steps.scope.outputs.run == 'true'
+        name: Scenarios
+        working-directory: desktop/frontend-next
+        env:
+          RELAY_HOST_BIN: ${{ github.workspace }}/desktop/frontend-next/relay/bin/studio-host
+          RELAY_FAKE_BIN: ${{ github.workspace }}/desktop/frontend-next/relay/bin/relayfake
+          RELAY_DIST: ${{ github.workspace }}/desktop/frontend-next/dist-relay
+        run: node relay/live.mjs
+
   # A pull request with no platform-relevant change (scripts/ci-platform-scope.sh)
   # moves the Windows and macOS legs onto an ubuntu runner with every step
   # skipped, so the check names still report without a scarce runner. A push to