fix(desktop): avoid native HiDPI double scaling

[Luhaozhu]

What changed

• keep the adaptive DPI compensation for Windows WebView2
• use page zoom 1.0 for native macOS and Linux WebViews
• cover both Windows compensation and native HiDPI behavior with focused unit tests

Root cause

The desktop shell forces the WebView2 device scale factor to 1 to avoid fractional-DPI popup positioning issues on Windows, then restores the intended size with set_zoom. That compensation was applied on every platform. Native macOS and Linux WebViews already honor the system scale factor, so Retina/HiDPI displays received a second page zoom. On a 3024x1964 Retina display this produced approximately 1.18x scaling.

Impact

macOS and Linux now rely on their native HiDPI rendering, reducing the oversized desktop UI without changing web frontend CSS. Windows keeps its existing high-DPI and remote-desktop behavior.

Validation

• rustfmt --edition 2021 --config skip_children=true --check desktop/src-tauri/src/lib.rs
• focused upstream Rust suite: 4 display scaling tests passed
• Desktop CI: scripts plus Linux, Windows, macOS Apple Silicon, and macOS Intel offline cold-start jobs passed
• Desktop Release rebuild: all four platform packages built and uploaded successfully
• Desktop 0.2.8 draft release contains both aarch64 and x64 macOS DMGs