Canvas rendering is blurry on Retina/HiDPI displays

Describe the bug
On Retina/HiDPI displays, the main drawing canvas appears to be rendered at CSS-pixel resolution instead of device-pixel resolution, so the editor output looks softer/blurry compared with what you would expect on a DPR > 1 screen.

To Reproduce
Steps to reproduce the behavior:

1. Open Klecks on a Retina / HiDPI display (for example a MacBook display with devicePixelRatio > 1).
2. Create or open a project.
3. Look at the canvas at normal zoom / fit-to-screen.
4. The rendered image looks softer than expected, as if the viewport canvas is not using the full native backing-store resolution.

Expected behavior
The viewport canvas should size its backing store using devicePixelRatio (or equivalent HiDPI handling) and render sharply on Retina / HiDPI screens.

Screenshots / Videos
Not attached.

Please complete the following information:

• Device: Desktop / MacBook with Retina display
• OS: macOS
• Browser: Chrome / Safari / Firefox
• App version: current main (d80d91d) / current hosted app

Additional context
A quick code check suggests HiDPI support is only partially wired up:

• src/app/script/klecks/ui/project-viewport/project-viewport.ts already has useNativeResolution and scales resFactor from devicePixelRatio.
• But the main editor viewport in src/app/script/klecks/ui/easel/easel.ts creates new ProjectViewport(...) without enabling useNativeResolution.
• src/app/script/klecks/ui/project-viewport/preview.ts also explicitly passes useNativeResolution: false.

So it looks like the canvas backing store stays at CSS-pixel resolution in the main editor / preview paths, which would explain the blur on Retina displays.

If the lower-resolution path is intentional for performance reasons, it might still be worth exposing native-resolution rendering as an option.

[bitbof]

Thanks for reporting and providing extensive details. I am actually working on high DPI support at the moment. It is currently disabled because of performance concerns. With the upcoming improvements it should be able to handle it and be enabled by default.