Always-visible thread tally per project (in-app strip + tray)

[arogan178]

Problem. With threads across several projects, anything outside the visible part of the sidebar is easy to forget. Examples: a finished thread in a collapsed project, or one waiting on me further down the list. The "Completed" pill and "Mark unread" only help when that row is on screen. The taskbar badge resets when the window gets focus, so one glance clears it while the work is still there.

Proposal. A compact tally, like a population counter in a strategy game:

• In app: one line at the top of the sidebar: each project name followed by counts, using the existing status dots (running, waiting on me, finished but unread). Clicking a project jumps to its first thread that needs attention.
• Outside the app: a tray icon with the total. Its tooltip lists projects in plain text, e.g. oskel: 1 running · Spinta: 1 waiting, 2 unread.
• Optional: a shortcut that cycles through the threads that need me (waiting first, then unread).

Counts use states the sidebar already computes, skip subagent, settled, snoozed and archived threads, and don't reset on focus. Off by default behind a setting.

Related: #17394 reorganises the sidebar by project. This is a smaller, separate signal that works with either sidebar layout.