[Task] WebView2 rendering and cross-platform testing

[shellen]

Part of: [Epic] Add Windows support (#66)

Summary

Validate that the viewer HTML, self-hosted fonts, and overall UI render correctly in WebView2 (Edge-based) on Windows. This is a testing/QA task to catch rendering differences before release.

Test Areas

• Viewer HTML: Edge-based WebView2 vs macOS WKWebView rendering differences
• Font rendering: Self-hosted fonts (Work Sans, etc.) with ClearType vs macOS font smoothing
• Kokoro TTS: Local TTS model loading behavior on Windows
• System tray: Verify tray icon renders and menus work
• Notifications: Verify native notifications display correctly

Acceptance Criteria

• Viewer HTML renders correctly in WebView2
• Fonts display acceptably on Windows (ClearType)
• Kokoro TTS loads and plays audio on Windows
• No visual regressions compared to macOS build
• Basic smoke test passes on Windows 10 and Windows 11

Estimate: ~1 day