Combine critical/warning sliders into one range slider (Settings + Home)

[almothafar]

Closes #126.

Replaces the two separate warning/critical SeekBarPreferences with a single two-thumb Material RangeSlider — left thumb = critical, right thumb = warning — shown both in Settings and as an in-fly control on the home screen.

What changed

New

• ui/preference/BatteryRangeSliderHelper.java — single source of truth for bounds (10–50), integer step, 5% minimum thumb separation, defaults (critical 20 / warning 40), and the N% label formatter. Also the clampPair(…) logic that keeps any stored/corrupt pair inside bounds and correctly ordered.
• ui/preference/BatteryRangeSliderPreference.java — a non-persistent Preference hosting the slider. It writes each thumb to the existing key_critical_battery_level / key_warn_battery_level keys, so BatteryLevelReceiver and the gauge read the same values with no logic change.
• res/layout/preference_battery_range_slider.xml — title + slider + colored end captions.

Changed

• res/xml/pref_general.xml — the two SeekBarPreferences → the new preference.
• ui/fragment/GenericPreferenceFragment.java — removed the now-dead battery-level cross-field validation/summary code (the slider's minSeparation enforces critical < warning structurally). Temperature-threshold handling is untouched.
• res/layout/activity_main.xml + ui/MainActivity.java — in-fly RangeSlider on the portrait home screen. Persists on release, refreshes the gauge live, and re-syncs from preferences on resume so Settings and Home stay in agreement. (Landscape omitted — the details fragment fills the width there.)
• Strings for the captions/title in EN + AR; removed the two unused validation error strings.

Design decisions (from #126)

• Range 10–50, step 1, min separation 5 — union of the old bands, "not 0–100".
• Neutral thumbs + colored captions: stock RangeSlider can't color thumbs individually, so the red Critical / amber Warning captions carry the color coding (both colors already in colors.xml).
• Value display: floating bubble while dragging plus always-on captions that update live.

Testing

• Added BatteryRangeSliderHelperTest — a parameterized test for the representative clamp cases plus an exhaustive sweep asserting every input yields an in-range, ≥5-apart, correctly-ordered pair (matches the repo's boundary-test style).
• Verified the clamp math independently over an exhaustive input sweep; validated all changed XML for well-formedness.
• Note: this environment has no Android SDK, so the full Gradle build/instrumentation runs in CI rather than locally.

Notes

• Time range was raised in discussion — it already exists as the quiet-hours window (start/end TimePickerPreferences). No change made there; flagged only for completeness.

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