[FEATURE]: Taskbar state indicators - running / awaiting confirmation / completed

Feature hasn't been suggested before.

• I have verified this feature hasn't been suggested before. Related but different scope than #43937 (see below).

Describe the enhancement

When OpenCode runs long tasks, users often switch to other windows while the agent works in the background. There is currently no way to tell from the taskbar alone whether OpenCode is still working, waiting for user confirmation (permission prompt / question), or has finished.

Requesting three distinct visual states on the taskbar icon:

This lets users keep working in other windows and glance at the taskbar to catch state changes immediately, without switching back and forth.

Suggested implementation paths

• Windows Desktop (Electron)
  • app.setProgressBar(win, { mode: 'indeterminate' }) → running
  • win.setOverlayIcon() → awaiting-confirmation / completed overlay badges
  • optional: win.flashFrame(true) when input is needed
• TUI
  • Emit OSC 9;4 progress sequences so terminals like Windows Terminal map them onto the native taskbar progress ring (indeterminate = running; error/paused = needs attention); no-op on terminals without support
• Linux
  • Unity launcher progress (launcher.setProgress()) and dock badging where available (libunity/DockManager); graceful fallback on DEs without support
• (optional) macOS: app.dock.setBadge() + dock.bounce()

Related issues

• #43937 — numeric count of sessions needing attention on the Windows taskbar. This request is broader/complementary: distinct tri-state visuals including running and completed states, across Desktop and TUI.
• #24807 — terminal-native progress indicator (closed, related TUI side)

中文说明

当 OpenCode 执行长任务时，用户通常会切到其他窗口工作。目前无法仅凭任务栏判断 OpenCode 是正在运行、等待用户确认（权限请求/提问）还是已完成。

希望任务栏图标呈现三种状态：

这样后台跑任务时，瞟一眼任务栏即可掌握状态变化，前台工作完全不受影响。

实现建议见上方英文部分（Electron 任务栏 API / OSC 9;4 终端进度序列 / Linux launcher progress，均带优雅降级）。