Open message links externally

[yibo365]

Summary

• route markdown-created web links through Electron's external browser handling
• prevent external URLs from opening inside the Pi window or creating extra Electron windows
• add a core Electron spec covering message link clicks

Fixes #20

Verification

• pnpm --filter @pi-gui/desktop typecheck
• pnpm --dir apps/desktop exec electron-vite build
• pnpm --filter @pi-gui/desktop run test:e2e:runner -- apps/desktop/tests/core/external-links.spec.ts
• git diff --check

Note: pnpm --filter @pi-gui/desktop build is currently blocked in my local environment by xcrun swiftc failing to load the Xcode toolchain while building the notification helper.

[vercel]

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