fix(notify): use ActivityWatch logo as notification small icon

[0xbrayo]

Problem

The foreground-service notification (BackgroundService) and activity-time alerts (NotifyWorker) used R.mipmap.aw_launcher_round as setSmallIcon().

On API 26+ that resource is an adaptive-icon XML. Status-bar icons are alpha-only, so Android rendered a white blob or fell back to the default robot — not the ActivityWatch logo.

Fix

Add a white-on-transparent ic_stat_notification drawable (official AW clock / A / W mark, all densities) and use it as the small icon in both notification builders.

Change

- .setSmallIcon(R.mipmap.aw_launcher_round)
+ .setSmallIcon(R.drawable.ic_stat_notification)

Closes #226

[greptile-apps]

Greptile Summary

The PR replaces adaptive launcher icons with dedicated notification-status icons for the foreground service and activity-time alerts.

• Updates both notification builders to reference R.drawable.ic_stat_notification.
• Adds correctly scaled PNG resources for mdpi through xxxhdpi.

Confidence Score: 5/5

The PR appears safe to merge with no actionable defects identified.

The new drawable is available to all build variants, covers every standard density, uses valid density-appropriate PNG dimensions, and is a supported small-icon resource throughout the app's API range.

Important Files Changed

Filename	Overview
mobile/src/main/java/net/activitywatch/android/BackgroundService.kt	Uses the new dedicated drawable for the lifecycle-critical foreground-service notification; no issue identified.
mobile/src/main/java/net/activitywatch/android/workers/NotifyWorker.kt	Uses the same dedicated drawable for activity-threshold notifications; no issue identified.
mobile/src/main/res/drawable-mdpi/ic_stat_notification.png	Adds a valid 24×24 RGBA notification icon for mdpi.
mobile/src/main/res/drawable-hdpi/ic_stat_notification.png	Adds a valid 36×36 RGBA notification icon for hdpi.
mobile/src/main/res/drawable-xhdpi/ic_stat_notification.png	Adds a valid 48×48 RGBA notification icon for xhdpi.
mobile/src/main/res/drawable-xxhdpi/ic_stat_notification.png	Adds a valid 72×72 RGBA notification icon for xxhdpi.
mobile/src/main/res/drawable-xxxhdpi/ic_stat_notification.png	Adds a valid 96×96 RGBA notification icon for xxxhdpi.

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