fix(wallpaper): improve setting reliability

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Summary

• stream wallpaper downloads to unique temporary files instead of decoding full-size bitmaps
• preserve the active wallpaper when refresh or persistence fails
• move automatic wallpaper setting behind an injected IO-safe WallpaperSetter
• guard unsupported and policy-disabled wallpaper operations
• remove unsafe wallpaper state access and harden the system crop flow
• sample notification artwork to reduce memory pressure
• preserve coroutine cancellation and add a dedicated FileProvider

Verification

• ./gradlew clean :app:assembleDebug spotlessCheck --warning-mode all
• ./gradlew :app:lintDebug

Manual testing

See the PR conversation/checklist provided separately.

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Manual test checklist

Core refresh and persistence

• Launch with an existing wallpaper and press Refresh; the new image loads without a crash.
• Enable automatic setting, press Refresh, and verify the home-screen wallpaper changes and the success message appears.
• Disable automatic setting, press Refresh, and verify the in-app image changes but the device wallpaper does not.
• Force-stop and reopen the app after a successful refresh; verify the same image is restored.

Failure recovery

• Enable airplane mode, press Refresh, and verify the existing image remains usable and the app does not crash.
• Restore connectivity and refresh again; verify recovery without clearing app data.
• Tap Refresh two or three times quickly; verify the final image remains valid and can still be set.

Manual crop flow

• Press the Set Wallpaper button and verify the system crop/picker opens.
• Complete the picker and verify the wallpaper is applied.
• Open it again and cancel; verify the app resumes normally.
• If available, repeat on a second OEM/device because picker behavior is vendor-specific.

Notifications/background refresh

• Grant notification permission and trigger RefreshWallpaperWorker using Android Studio Background Task Inspector (or wait for its interval).
• Verify the rich notification appears with thumbnail and expanded artwork.
• Tap it and verify the app opens with the downloaded wallpaper available.
• Deny notification permission and trigger the worker again; verify there is no notification or crash.

Device restrictions and memory

• If available, test under a work profile/device policy that disallows wallpaper changes; expect an error message rather than a crash.
• Repeatedly refresh on a high-resolution device while watching Memory Profiler; memory should not spike from decoding the full raw wallpaper.

Automatic setting now explicitly targets the home-screen wallpaper. Lock-screen behavior can still vary when an OEM links lock and system wallpapers.