feat: add device store

- Selected device state
- Custom device management
- Device selection by ID

Co-Authored-By: Claude <noreply@anthropic.com>

src/lib/stores/device.svelte.ts

@@ -0,0 +1,57 @@
+import { PRESET_DEVICES, DEFAULT_DEVICE } from '$lib/data/devices';
+import type { Device, CustomDeviceFormData } from '$lib/types';
+
+function createDeviceStore() {
+	let selected = $state<Device>(DEFAULT_DEVICE);
+	let customDevices = $state<Device[]>([]);
+
+	const allDevices = $derived<Device[]>([...PRESET_DEVICES, ...customDevices]);
+
+	function selectDevice(id: string): void {
+		const device = allDevices.find((d) => d.id === id);
+		if (device) {
+			selected = device;
+		}
+	}
+
+	function addCustomDevice(formData: CustomDeviceFormData): Device {
+		const device: Device = {
+			id: `custom_${formData.width}x${formData.height}`,
+			name: formData.name,
+			brand: 'Custom',
+			width: formData.width,
+			height: formData.height,
+			greyLevels: formData.greyLevels,
+			outputFormat: formData.outputFormat
+		};
+
+		customDevices = [...customDevices, device];
+		selected = device;
+
+		return device;
+	}
+
+	function removeCustomDevice(id: string): void {
+		customDevices = customDevices.filter((d) => d.id !== id);
+		if (selected.id === id) {
+			selected = DEFAULT_DEVICE;
+		}
+	}
+
+	return {
+		get selected() {
+			return selected;
+		},
+		get customDevices() {
+			return customDevices;
+		},
+		get allDevices() {
+			return allDevices;
+		},
+		selectDevice,
+		addCustomDevice,
+		removeCustomDevice
+	};
+}
+
+export const deviceStore = createDeviceStore();