Redesign macOS dock icon

[debba]

The old macOS dock icon was the bare isometric cube on a transparent background. On Tahoe (and honestly already on Sonoma/Sequoia) that looks out of place next to system apps that all sit inside a proper squircle — the cube floats, has no glass treatment, and on light wallpapers the dark edges fight the dock.

This PR replaces only src-tauri/icons/icon.icns with a new design:

• True Apple squircle (continuous-curvature superellipse, not a rounded rect)
• Light glass background with a top sheen and a faint inner edge shadow
• Very subtle teal and violet auroras in opposite corners, picking up the cube's own gradient colors so the icon still reads as Tabularis at a glance
• Cube logo centered, slightly scaled down, with a soft drop shadow to lift it off the surface

The source SVG lives at public/logo-macos.svg so it can be re-rendered if we ever want to tweak it.

Nothing else changes: icon.ico (Windows), icon.png and the square PNGs (Linux / Tauri default), and the iOS/Android folders are untouched, so other platforms keep the existing mark. No tauri.conf.json change either — icon.icns was already the macOS path.

Test plan

• pnpm tauri build on macOS produces an .app whose Finder/Dock icon shows the new squircle
• At 16x16 (menu bar / spotlight) the cube is still recognizable
• Windows and Linux builds still show the original cube