feat(renderer): track devicePixelRatio changes at runtime

[sauyon]

The renderer captured window.devicePixelRatio once in its constructor and never re-checked it. Browser zoom, window drags between monitors with different scales, and OS scale changes all leave the canvas backing store sized to the old ratio — blurry one way, oversharp the other.

Subscribe to a matchMedia query pinned to the current DPR and update the field on change. Since each MediaQueryList is bound to one ratio, re-create the query on every change. The render loop's needsResize check picks up the new DPR and forces a full resize+redraw on the next frame, so no explicit render call is needed.

Skip the subscription when the caller pinned a DPR via options — they opted out of browser-driven changes.

[gemini-code-assist]

Code Review

This pull request introduces automatic tracking of the device pixel ratio (DPR) in the CanvasRenderer to handle browser-driven changes like zooming or monitor dragging. It adds methods to observe and unobserve DPR changes using matchMedia, along with comprehensive unit tests to verify the behavior. The reviewer suggested improving the robustness of the observeDevicePixelRatio method by ensuring existing listeners are cleaned up before re-subscription and adding a guard to prevent late-firing events after disposal, providing a code snippet to implement these safety checks.