feat(player): add adaptive brightness mode support

Extend the brightness slider to allow values below 0% which triggers
adaptive/auto brightness mode (screenBrightness = -1.0f). The brightness
indicator shows 'Auto' with an empty bar when in adaptive mode.

- Widen gesture range from [0,1] to [-1,1] so dragging below 0 enters auto mode
- readCurrentBrightness returns -1f instead of 0.5f when system auto is active
- applyBrightness writes -1f to LayoutParams when value < 0
- PlayerAdjustmentIndicator displays 'Auto' in tertiary color for negative values

app/src/main/java/hu/bbara/purefin/ui/screen/player/PlayerScreen.kt

@@ -148,7 +148,7 @@ fun PlayerScreen(
             onDoubleTapCenter = { viewModel.togglePlayPause() },
             onVerticalDragLeft = { delta ->
                 val diff = (-delta / 800f)
-                brightness = (brightness + diff).coerceIn(0f, 1f)
+                brightness = (brightness + diff).coerceIn(-1f, 1f)
                 applyBrightness(activity, brightness)
             },
             onVerticalDragRight = { delta ->
@@ -400,12 +400,12 @@ private fun formatSeekDelta(deltaMs: Long): String {
 
 private fun readCurrentBrightness(activity: Activity?): Float {
     val current = activity?.window?.attributes?.screenBrightness
-    return if (current != null && current >= 0) current else 0.5f
+    return if (current != null && current >= 0) current else -1f
 }
 
 private fun applyBrightness(activity: Activity?, value: Float) {
     activity ?: return
     val params = activity.window.attributes
-    params.screenBrightness = value
+    params.screenBrightness = if (value < 0f) -1f else value
     activity.window.attributes = params
 }

app/src/main/java/hu/bbara/purefin/ui/screen/player/components/PlayerAdjustmentIndicator.kt

@@ -31,7 +31,8 @@ fun PlayerAdjustmentIndicator(
     sliderHeight: Dp = 140.dp,
 ) {
     val scheme = MaterialTheme.colorScheme
-    val percent = (value.coerceIn(0f, 1f) * 100).roundToInt()
+    val isAuto = value < 0f
+    val percent = if (isAuto) 0 else (value * 100).roundToInt()
     val clamped = value.coerceIn(0f, 1f)
 
     Box(
@@ -68,9 +69,10 @@ fun PlayerAdjustmentIndicator(
                 )
             }
             Spacer(modifier = Modifier.height(16.dp))
+            val label = if (isAuto) "Auto" else "$percent%"
             Text(
-                text = "$percent%",
-                color = scheme.onSurface,
+                text = label,
+                color = if (isAuto) scheme.tertiary else scheme.onSurface,
                 style = MaterialTheme.typography.titleMedium
             )
         }