feat: animate overlay visibility changes

[steipete]

Summary

• Fade overlay master visibility and individual frame, tag, and watermark changes on macOS, Windows, and Linux.
• Keep overlay layers persistent while animating so rapid menu toggles settle correctly and canonical stacking stays stable.
• Respect each platform's reduced-motion setting and keep startup/display rebuilds immediate.

Test plan

• macOS: 62 Swift tests; signed debug app; real status-menu toggles for master and individual layers; rapid reversal and unaffected-layer checks.
• Windows 11 VM: build with zero warnings/errors; 26 tests; format verification; live fade-in/fade-out capture.
• Ubuntu VM: native build; 13 workspace tests; format verification; X11/Openbox/Picom live fade-in/fade-out capture.
• Autoreview: clean.

Visual proof

Safe headless Linux before/after proof will be attached below. Desktop captures from macOS and Windows were not uploaded because they contained private desktop data.

[steipete]

Safe headless visual proof

Before (tag hidden):

After (tag visible):