feat: add maximized file panel layout

[looluo]

Summary

• add a maximize/restore control to the open file panel header, immediately left of the panel visibility control
• let the file panel replace the conversation area while keeping the conversation mounted so active agent runs continue
• preserve sidebar controls, stored panel width, responsive behavior, blocking-attention recovery, and keyboard focus
• add English/Chinese labels and focused regression coverage

Testing

• node_modules/.bin/tsc --noEmit
• npm run lint
• node --test components/AppShell.file-panel-maximize.test.mjs components/AppShell.file-viewer-state.test.mjs components/AppShell.mobile-toolbar.test.mjs components/MobilePwaLayout.test.mjs (18 passed)
• browser QA at 1264px, 800px, and 600px, including maximize/restore focus transfer and responsive layout transitions

Full-suite note

npm test was run, but the current baseline has unrelated failures in component render tests that call useI18n without an I18nProvider, plus a Git Bash PATH-separator assertion in lib/project-command-env.test.mjs.