HUD overlay: switch from fully rounded to squircle (or sharp edges) for consistent cross-platform rendering

[michTheBrandofficial]

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The current fully-rounded corners on the HUD overlay suffer from subtle clipping/sharp edges on all Webview-based runtimes like this one (due to border-radius rounding inconsistency). This creates visual inconsistencies that stand out. Especially to the trained design eye.	Switching to a squircle shape (or sharp edges as an option) ensures smoother, more consistent rendering across all platforms. Squircles feel modern and premium (like macOS system UI), while avoiding the rounding artifacts entirely.

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This small tweak brings better visual harmony to the entire HUD and aligns with the app's existing design language, making OpenScreen feel even more polished without changing functionality or user experience for most people.

Changes for this are underway in #450. @siddharthvaddem let me know what you think about this.