fix(desktop): avoid linux hidden titlebar freeze

[tt-a1i]

Summary

• Use native system titlebars on Linux desktop windows instead of Electron titleBarStyle: hidden / titleBarOverlay.
• Gate renderer -webkit-app-region:drag strips when Linux uses native titlebars, covering the main shell, secondary chat windows, and route overlays.
• Add pure Electron-free coverage for the desktop window chrome policy across Linux, macOS, and Windows.

Fixes #48515.

Validation

• cd apps/desktop && node --test electron/window-chrome.test.cjs
• npm run typecheck --workspace apps/desktop
• cd apps/desktop && npx eslint electron/main.cjs electron/window-chrome.cjs electron/window-chrome.test.cjs src/app/shell/app-shell.tsx src/app/shell/titlebar.ts src/store/updates.test.ts src/components/assistant-ui/thread-list.tsx src/app/overlays/overlay-view.tsx
• cd apps/desktop && npx prettier --check electron/window-chrome.cjs electron/window-chrome.test.cjs src/app/shell/app-shell.tsx src/app/shell/titlebar.ts src/app/overlays/overlay-view.tsx src/components/assistant-ui/thread-list.tsx src/global.d.ts src/store/updates.test.ts package.json
• npm run test:ui --workspace apps/desktop -- src/app/shell/titlebar.test.ts src/store/updates.test.ts
• git diff --check

Notes

• npm run test:desktop:platforms --workspace apps/desktop still fails on an existing unrelated static-scan expectation in electron/windows-child-process.test.cjs: it looks for the literal substring execFileSync(pyExe, while current main.cjs has that call split over multiple lines on origin/main.
• I could not run a live Ubuntu 22.04 GNOME/X11 smoke test in this environment.