fix: warn before closing when any tab is unsaved

[imwyvern]

The window close guard was using the active tab's modified state as the saved/unsaved signal for the whole BrowserWindow. Switching from an edited markdown tab to a saved tab could mark the window as saved, so closing skipped the existing save confirmation flow.

The save-state watcher now reports whether any tab in the window is modified, keeping the main-process window state dirty until all tabs are saved or discarded.

Verification:

• pnpm run build
• pnpm exec oxlint src/renderer/hooks/useContent.ts && pnpm exec oxfmt --check src/renderer/hooks/useContent.ts

Fixes #200

[LarryZhu-dev]

Hi，感谢你的贡献，我正在休假，可能会在稍晚查看此请求。

[github-actions]

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