[codex] feat(devices): describe power saving modes

[TobiasRoeddiger]

Summary

• Add short descriptions for firmware power saving modes in the device detail and stereo pair selectors.
• Use mode-specific icons for Off, Minimal, Balanced, and Aggressive.
• Hide the stereo pair power saving section when neither side supports it, matching device detail behavior.
• Update the connector activity tooltip test to match the current network-unavailable copy.

Validation

• dart format lib/widgets/devices/device_detail/power_saving_mode_widget.dart lib/widgets/devices/devices_page.dart
• flutter test test/widgets/connector_activity_indicator_test.dart
• flutter analyze
• flutter test

Notes

• Manually installed and launched the debug build on Pixel 9a while testing the UI.

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Visit the preview URL for this PR (updated for commit d7331fc):

https://open-earable-web--pr272-codex-power-saving-m-p49va4nl.web.app

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PR Build Artifacts

• Android APK: Download Android APK
• Linux Bundle: Download Linux Bundle
• Windows Runner: Download Windows Runner
• Web Bundle: Download Web Bundle

Full workflow run: https://github.com/OpenEarable/app/actions/runs/28088474653