[Desktop] Titlebar tab hover/selection switches instantly (no transition)

Description

Titlebar session tabs switch their background instantly between idle, hover, active and pressed states, and the separators between tabs toggle with display: none. There is no transition, so hover/selection feels like a hard cut.

Proposal: transition the tab background over 140ms with an ease-out curve (registering --tab-base/--tab-overlay as colors so they interpolate) and fade the separators via opacity instead of toggling display. Measured in the packaged desktop app (forcing hover via the DevTools protocol):

• before: rgb(8,8,8) → rgb(46,46,46) in under 30ms (instant)
• after: rgb(8,8,8) → rgb(35,35,35) at +30ms → rgb(46,46,46) at +90ms
• separator: opacity 1 → 0.25 at +30ms → 0 at +240ms (was a display toggle)

Plugins

none

OpenCode version

OpenCode Desktop 1.18.29

Steps to reproduce

1. Open two or more session tabs
2. Hover the inactive tab or click to switch
3. The background and the separators change instantly, with no transition

Screenshot and/or share link

No screenshot (motion change).

Operating System

CachyOS (Arch Linux), KDE Plasma 6.7.5, Wayland

Terminal

n/a (desktop app)