fix(e2e-chat): dynamic canvas port to avoid conflict with Gitea :3000

The operator host runs Gitea on 127.0.0.1:3000. With act_runner using
container.network: host, the E2E Chat job's Next.js dev server (also
port 3000) collides and crashes with EADDRINUSE.

Changes:
- Pick an ephemeral host port for the canvas dev server (same pattern
  already used for the platform port).
- Pass the port to next dev via -p flag (overrides package.json -p 3000).
- Update the health-check loop to probe the dynamic port.
- Export PLAYWRIGHT_BASE_URL so Playwright tests connect to the right URL.
- Make playwright.config.ts read baseURL from PLAYWRIGHT_BASE_URL env var
  with fallback to localhost:3000 (preserves local dev workflow).

This is an infrastructure compatibility fix, not a test logic change.

.gitea/workflows/e2e-chat.yml

@@ -210,16 +210,29 @@ jobs:
         working-directory: canvas
         run: npx playwright install --with-deps chromium
 
+      - name: Pick canvas port
+        if: needs.detect-changes.outputs.chat == 'true'
+        run: |
+          CANVAS_PORT=$(python3 - <<'PY'
+          import socket
+          with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+              s.bind(("127.0.0.1", 0))
+              print(s.getsockname()[1])
+          PY
+          )
+          echo "CANVAS_PORT=${CANVAS_PORT}" >> "$GITHUB_ENV"
+          echo "Canvas host port: ${CANVAS_PORT}"
+
       - name: Start canvas dev server (background)
         if: needs.detect-changes.outputs.chat == 'true'
         working-directory: canvas
         run: |
           export NEXT_PUBLIC_PLATFORM_URL="http://127.0.0.1:${PLATFORM_PORT}"
           export NEXT_PUBLIC_WS_URL="ws://127.0.0.1:${PLATFORM_PORT}/ws"
-          npm run dev > canvas.log 2>&1 &
+          npx next dev --turbopack -p "${CANVAS_PORT}" > canvas.log 2>&1 &
           echo $! > canvas.pid
           for i in $(seq 1 30); do
-            if curl -sf http://localhost:3000 > /dev/null 2>&1; then
+            if curl -sf "http://localhost:${CANVAS_PORT}" > /dev/null 2>&1; then
               echo "Canvas up after ${i}s"
               exit 0
             fi
@@ -235,6 +248,7 @@ jobs:
         run: |
           export E2E_PLATFORM_URL="http://127.0.0.1:${PLATFORM_PORT}"
           export E2E_DATABASE_URL="${DATABASE_URL}"
+          export PLAYWRIGHT_BASE_URL="http://localhost:${CANVAS_PORT}"
           npx playwright test e2e/chat-desktop.spec.ts e2e/chat-mobile.spec.ts
 
       - name: Dump platform log on failure

canvas/playwright.config.ts

@@ -8,7 +8,7 @@ export default defineConfig({
   workers: 1,
   retries: 0,
   use: {
-    baseURL: "http://localhost:3000",
+    baseURL: process.env.PLAYWRIGHT_BASE_URL || "http://localhost:3000",
     headless: true,
     screenshot: "only-on-failure",
   },