Bounce the Dock icon on a new notification

[kuznetsov-konstantin]

Summary

When a session posts a notification while agterm is in the background, it
would be great if the app also bounced its Dock icon, in addition to the
existing banner and sidebar count badge.

Motivation

The whole point of agterm is running many long-lived agent sessions at once
and knowing which one needs you. When agterm isn't the frontmost app, a
banner is easy to miss and the sidebar badge isn't visible at all. A
bouncing Dock icon is the standard macOS signal for "something over here
wants your attention," and it fits naturally with the existing attention
model of status glyphs and notification badges.

Proposed behavior

• Bounce the Dock icon when a notification arrives for a session that isn't
  currently focused.
• Don't bounce when agterm is already the active app, the same way a
  notification from the already-focused pane is suppressed today.
• Ideally a toggle in General settings, alongside the existing "Show
  notification banners" and "Show notification badges" options, so it can be
  turned off by anyone who doesn't want it.

[umputun]

running many long-lived agent sessions is one of agterm's goals, not the whole point, but the need behind this is real, when agterm isn't frontmost a banner is easy to miss.

personally I'm not a fan of bouncing dock icons, but you've got a point, it can be useful. So I added it, off by default: Settings ▸ Notifications ▸ Bounce Dock icon, with three modes, None (the default), Once (a single bounce), or Until focused (keeps bouncing until you switch to agterm). It only fires when a notification arrives while agterm is in the background.

merged in #215.