feat: add native macOS zoom controls

[melchoir55]

Summary

• Add Zoom In, Zoom Out, and Actual Size to the macOS View menu with standard keyboard shortcuts.
• Scale and reflow the whole interface through WKWebView.pageZoom, with persistent 50%-300% bounds.
• Keep the custom title bar, traffic-light clearance, and clickable hit regions aligned at every zoom level.
• Leave browser and non-macOS shell behavior unchanged, and document the native integration.

Validation

• 225 frontend tests passed.
• Svelte check completed with no diagnostics.
• Production frontend build passed.
• go test ./...
• CGO_ENABLED=1 go test -tags webview ./cmd/webview
• go vet ./...
• CGO_ENABLED=1 go vet -tags webview ./cmd/webview
• Live macOS bundle verified at 50%, 100%, and 300%, including menu boundary states, title-bar control click-through, Actual Size, and relaunch persistence.
• git diff --check

[rengwu]

thanks for the PR @melchoir55, this is a welcomed enhancement. will go through this soon.