Fix fresh-agent header in narrow panes

[danshapiro]

Summary

• keep fresh-agent identity and cwd metadata visible in narrow split pane headers
• hide optional fresh-agent header actions in cramped pane headers before they crowd out labels
• add a browser regression for a narrow split freshcodex pane

TDD

• Red: new Playwright regression failed because the fresh-agent cwd metadata span was hidden in a 320px viewport split pane
• Green: added pane-header container rules and compact fresh-agent header styling
• Refactor: suppressed fresh-agent network effects in the regression so it only covers header layout

Verification

• npm run test:e2e:chromium -- test/e2e-browser/specs/fresh-agent.spec.ts --grep "fresh-agent header keeps identity visible"
• npm run test:vitest -- --run test/unit/client/components/panes/PaneHeader.test.tsx test/unit/client/components/panes/PaneContainer.test.tsx
• npm run typecheck:client
• npm run lint
• npm run test:e2e:chromium -- test/e2e-browser/specs/fresh-agent.spec.ts
• FRESHELL_TEST_SUMMARY="fresh-agent narrow header identity followup" npm run check