Keep Playwright's own artifacts out of the lint

A checking lint reports what a writing one silently reformatted: after any test
run, `playwright/.auth/user.json` and `test-results/.last-run.json` are written
by Playwright and failed `make lint`. Both are gitignored, and `playwright-report`
was already excluded beside them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uq8mtNb97A7praJLyeEYgs

frontend/biome.json

@@ -10,6 +10,8 @@
       "!**/src/client/**/*",
       "!**/src/components/ui/**/*",
       "!**/playwright-report",
+      "!**/playwright/.auth",
+      "!**/test-results",
       "!**/playwright.config.ts"
     ]
   },