refactor(ui): extract shared ExposedDropdownField (issue #796) (#815)

DropdownField (ConfigScreen) and PluginProviderDropdown (PluginsScreen)
were byte-for-byte identical. Extract into ui/common/ExposedDropdownField
and route both call sites to it. No behavior change.

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/ui/common/ExposedDropdownField.kt

@@ -0,0 +1,69 @@
+package com.m57.hermescontrol.ui.common
+
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material3.DropdownMenuItem
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.ExposedDropdownMenuAnchorType
+import androidx.compose.material3.ExposedDropdownMenuBox
+import androidx.compose.material3.ExposedDropdownMenuDefaults
+import androidx.compose.material3.OutlinedTextField
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+
+/**
+ * Shared dropdown field used by [com.m57.hermescontrol.ui.config.ConfigScreen]
+ * and [com.m57.hermescontrol.ui.plugins.PluginsScreen]. Extracted from the
+ * byte-for-byte identical private copies that previously lived in each screen
+ * (issue #796).
+ */
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+fun ExposedDropdownField(
+    label: String,
+    options: List<String>,
+    selectedValue: String,
+    onOptionSelected: (String) -> Unit,
+) {
+    var expanded by remember { mutableStateOf(false) }
+
+    ExposedDropdownMenuBox(
+        expanded = expanded,
+        onExpandedChange = { expanded = it },
+    ) {
+        OutlinedTextField(
+            value = selectedValue.ifEmpty { options.firstOrNull() ?: "" },
+            onValueChange = {},
+            readOnly = true,
+            modifier =
+                Modifier
+                    .fillMaxWidth()
+                    .menuAnchor(ExposedDropdownMenuAnchorType.PrimaryNotEditable),
+            label = { Text(label) },
+            trailingIcon = { ExposedDropdownMenuDefaults.TrailingIcon(expanded = expanded) },
+            singleLine = true,
+            shape = RoundedCornerShape(8.dp),
+        )
+
+        ExposedDropdownMenu(
+            expanded = expanded,
+            onDismissRequest = { expanded = false },
+        ) {
+            options.forEach { option ->
+                DropdownMenuItem(
+                    text = { Text(option) },
+                    onClick = {
+                        onOptionSelected(option)
+                        expanded = false
+                    },
+                )
+            }
+        }
+    }
+}