tui: pause animation ticks while the terminal is blurred

[dgageot]

Why

When docker-agent runs in a background tab while the agent is streaming, the bubbletea animation tick chain (14 fps spinner + cache invalidations + view rebuilds) keeps firing into a screen nobody is looking at. That's pure CPU waste.

What

Drop `animation.TickMsg` while the terminal is blurred and let the chain die. Re-arm it on the next `FocusMsg` via `animation.StartTick()` so spinners resume immediately when the user comes back. The existing Docker Desktop release/restore cycle (commit cc2dfcf) is preserved.

Why not just `tea.WithFPS(30)`?

Static FPS reduction halves the ~60 cheap goroutine wake-ups per second everywhere — well under 1 % CPU savings. This change targets the expensive work — full `Update` + `View` + cache invalidation per spinner tick — exactly when the user can't see it (background tab), and costs nothing when they're looking at the terminal (foreground stays at full responsiveness).

Implementation note

Two flags rather than one:

• `focused` — for the spurious-`FocusMsg` filter introduced by Docker Desktop's release/restore (must default to `false` so the first real `FocusMsg` is treated as a transition).
• `tickPaused` — for animation gating (must default to `false` so ticks flow at startup, before any focus event arrives — some terminals never send one).

They share most transitions but their initial values differ, so keeping them separate avoids cute-but-wrong overloading.

[derekmisler]

my almost-4-year-old laptop thanks you.

[docker-agent]

Assessment: 🟢 APPROVE

The PR cleanly decouples the animation tick-pause concern from the focus-tracking concern by introducing two boolean flags with different initialization semantics. The logic is correct:

• Startup safety: tickPaused starts false so ticks flow before any focus event arrives (terminals that never send FocusMsg work correctly).
• Spurious-FocusMsg filter: focused still correctly suppresses the spurious FocusMsg emitted by Docker Desktop's RestoreTerminal cycle.
• Tick resumption: On FocusMsg, StartTick() is only called when animation.HasActive() is true — if no animations are active at refocus time, the tick chain will be restarted by the first component that calls StartTickIfFirst(), which is the existing behavior.
• m.program guard: The removal of the m.program == nil early-return is safe because SetProgram is called synchronously before p.Run() in cmd/root/new.go, so m.program is always set before the event loop can deliver any messages.
• tea.Batch with empty slice: Returns a nil Cmd, which bubbletea handles correctly.

No bugs found in the changed code.