Pause the gauge pulse when the app is backgrounded, not just on destroy (#129)

[almothafar]

Fixes #129.

Problem

The low-battery/charging pulse on the circular gauge is an INFINITE ValueAnimator. It was only stopped when the view is destroyed (CircularProgressBar.onDetachedFromWindow) — not when the activity is backgrounded.

MainActivity.onPause() already stops the battery-polling timer for exactly this reason, but it never paused the pulse. So opening the app (e.g. at critical) and switching away without swiping it closed left the pulse invalidating ~60×/s on the main thread for a view nobody can see, as long as the screen stayed on.

Fix

Mirror the existing timer handling on both lifecycle sides:

• MainActivity.onPause → pausePulseAnimation()
• MainActivity.onPostResume → resumePulseAnimation() (restarts only if still charging or critical)

New public pausePulseAnimation() / resumePulseAnimation() on CircularProgressBar make the lifecycle intent explicit, rather than relying on the refresh loop to restart the pulse as a side effect.

Not changed

• Foreground behavior is identical — the gauge still breathes when the app is open at critical/charging, and still breathes after leaving and returning. This only removes wasted background work.
• No new user-facing strings, so no values-ar work needed.

Verification

• ✅ :app:compileDebugJavaWithJavac passes.
• On-device (pending): set the critical threshold above the current level so the gauge goes critical immediately, then confirm the pulse (1) shows when the app is open, and (2) still animates correctly after Home → other app → return. Same with the charger plugged in.

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