fix(ui): soften continuous corners to superellipse(1.1)

[AprilNEA]

Closes CODE-474. Follow-up to CODE-449 (#282).

Problem

corner-shape: squircle (= superellipse(2)) cuts into the 45° apex only 54% as deep as a circular corner at the same border-radius — rounded-xl 12px reads as ≈6.5px, 2xl 16px as ≈8.7px, 3xl 24px as ≈13px. The whole app visibly lost corner roundness.

Evidence

Pixel-level XOR fit inside our own Electron, reference = figma-squircle@1.1.0 at cornerSmoothing: 0.6 (the --smoothing-ios token in the apple-graph library these corners were adopted from):

curve	XOR vs iOS reference (px²)
round = superellipse(1.00)	509 (0.3% of the corner box)
superellipse(1.10)	1644
superellipse(1.50)	5489
squircle = superellipse(2.00)	8697 (17× round)

iOS's continuous corner is shape-wise almost a circle; its character comes from curvature-transition smoothing (G2), which the superellipse family cannot express — any smoothing there is paid for with a shallower apex. Even superellipse(1.01) is farther from the Apple curve than plain round.

Change

superellipse(1.1) (decision: keep a hint of the continuous transition; the strictly-faithful answer would be round). Keeps ~94% of the perceived radius — 0.7px shallower at 12px, imperceptible — while the @supports gate now tests the exact value in use. Fractional superellipse() support and the squircle ≡ superellipse(2) / round ≡ superellipse(1) mapping were verified empirically in the bundled Chromium.

[linear-code]

CODE-474