style(discovery): improve m3 ux of scan progress

- Moved the Start/Stop Scan button to a permanent `bottomBar` surface so it is always accessible without scrolling.
- Automatically collapse the configuration UI (Preset picker and Dwell time picker) when a scan begins, immediately bringing the 'Scan Progress' card to the top of the screen.

Author: jamesarich

feature/discovery/src/commonMain/kotlin/org/meshtastic/feature/discovery/ui/DiscoveryScanScreen.kt

@@ -108,45 +108,56 @@ fun DiscoveryScanScreen(
                 },
             )
         },
+        bottomBar = {
+            androidx.compose.material3.Surface(
+                modifier = Modifier.fillMaxWidth(),
+                color = MaterialTheme.colorScheme.surface,
+                tonalElevation = 8.dp,
+                shadowElevation = 8.dp,
+            ) {
+                androidx.compose.foundation.layout.Box(
+                    modifier = Modifier.padding(horizontal = CONTENT_PADDING, vertical = 16.dp),
+                ) {
+                    ScanButton(
+                        scanState = scanState,
+                        isConnected = isConnected,
+                        hasPresetsSelected = selectedPresets.isNotEmpty(),
+                        onStart = viewModel::startScan,
+                        onStop = viewModel::stopScan,
+                    )
+                }
+            }
+        },
     ) { padding ->
         val isScanning = scanState !is DiscoveryScanState.Idle
         LazyColumn(
             contentPadding = padding,
             verticalArrangement = Arrangement.spacedBy(SECTION_SPACING),
-            modifier = Modifier.fillMaxSize().padding(horizontal = CONTENT_PADDING),
+            modifier = Modifier.fillMaxSize().padding(horizontal = CONTENT_PADDING).padding(top = SECTION_SPACING),
         ) {
             // Connection warning
             if (!isConnected) {
                 item(key = "connection_warning") { ConnectionWarningCard() }
             }
 
-            // Preset picker
-            item(key = "preset_picker") {
-                PresetPickerCard(
-                    selectedPresets = selectedPresets,
-                    onTogglePreset = viewModel::togglePreset,
-                    enabled = !isScanning,
-                )
-            }
-
-            // Dwell time picker
-            item(key = "dwell_picker") {
-                DwellTimePicker(
-                    selectedMinutes = dwellMinutes,
-                    onMinutesSelected = viewModel::setDwellDuration,
-                    enabled = !isScanning,
-                )
-            }
+            if (!isScanning) {
+                // Preset picker
+                item(key = "preset_picker") {
+                    PresetPickerCard(
+                        selectedPresets = selectedPresets,
+                        onTogglePreset = viewModel::togglePreset,
+                        enabled = true,
+                    )
+                }
 
-            // Start / Stop button
-            item(key = "scan_button") {
-                ScanButton(
-                    scanState = scanState,
-                    isConnected = isConnected,
-                    hasPresetsSelected = selectedPresets.isNotEmpty(),
-                    onStart = viewModel::startScan,
-                    onStop = viewModel::stopScan,
-                )
+                // Dwell time picker
+                item(key = "dwell_picker") {
+                    DwellTimePicker(
+                        selectedMinutes = dwellMinutes,
+                        onMinutesSelected = viewModel::setDwellDuration,
+                        enabled = true,
+                    )
+                }
             }
 
             // Scan progress section