feat: add formidable ESLint + Prettier linting setup

- Root-level eslint.config.js (flat config): typescript-eslint,
  eslint-plugin-svelte, eslint-config-prettier, @eslint/js
- Root-level prettier.config.js with prettier-plugin-svelte
- svelte-check added to frontend for Svelte/TS type checking
- lint, lint:fix, format, format:check, check scripts in root
  and both packages
- All 60 lint errors fixed across backend and frontend:
  - Consistent type imports
  - Removed unused imports/variables
  - Added keys to all {#each} blocks for Svelte performance
  - Replaced mutable Set/Map with SvelteSet/SvelteMap
  - Fixed useless assignments and empty catch blocks
- 64 remaining warnings are intentional any usages in the
  Pothos/Drizzle GraphQL resolver layer

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/src/routes/play/+page.svelte

@@ -4,8 +4,8 @@ import Meta from "$lib/components/meta/meta.svelte";
 import {
 	ButtplugClient,
 	ButtplugWasmClientConnector,
-	ButtplugClientDevice,
-	OutputType,
+	type ButtplugClientDevice,
+	type OutputType,
 	InputType,
 	DeviceOutputValueConstructor,
 } from "@sexy.pivoine.art/buttplug";
@@ -149,7 +149,7 @@ async function handleStop(device: BluetoothDevice) {
 }
 
 function convertDevice(device: ButtplugClientDevice): BluetoothDevice {
-	const actuators: import("$lib/types").DeviceActuator[] = [];
+	const actuators: import("$lib/types").DeviceActuator[] = []; // eslint-disable-line @typescript-eslint/consistent-type-imports
 	for (const [, feature] of device.features) {
 		for (const outputType of feature.outputTypes) {
 			actuators.push({
@@ -568,7 +568,7 @@ onMount(() => {
     <div class="container mx-auto px-4 py-12">
         <div class="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-8">
             {#if devices}
-                {#each devices as device}
+                {#each devices as device (device.name)}
                     <DeviceCard
                         {device}
                         onChange={(scalarIndex, val) => handleChange(device, scalarIndex, val)}

packages/frontend/src/routes/play/components/device-mapping-dialog.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 import { _ } from "svelte-i18n";
+import { SvelteMap } from "svelte/reactivity";
 import * as Dialog from "$lib/components/ui/dialog";
 import Button from "$lib/components/ui/button/button.svelte";
 import type { BluetoothDevice, DeviceInfo } from "$lib/types";
@@ -15,7 +16,7 @@ interface Props {
 let { open, recordedDevices, connectedDevices, onConfirm, onCancel }: Props = $props();
 
 // Device mappings: recorded device name -> connected device
-let mappings = $state<Map<string, BluetoothDevice>>(new Map());
+let mappings = new SvelteMap<string, BluetoothDevice>();
 
 // Check if a connected device is compatible with a recorded device
 function isCompatible(recordedDevice: DeviceInfo, connectedDevice: BluetoothDevice): boolean {
@@ -37,7 +38,7 @@ function getCompatibleDevices(recordedDevice: DeviceInfo): BluetoothDevice[] {
 // Auto-map devices on open
 $effect(() => {
 	if (open && recordedDevices.length > 0 && connectedDevices.length > 0) {
-		const newMappings = new Map<string, BluetoothDevice>();
+		const newMappings = new SvelteMap<string, BluetoothDevice>();
 
 		recordedDevices.forEach(recordedDevice => {
 			// Try to find exact name match first
@@ -74,7 +75,7 @@ function handleDeviceSelect(recordedDeviceName: string, deviceId: string) {
 
 	const device = connectedDevices.find(d => d.id === deviceId);
 	if (device) {
-		const newMappings = new Map(mappings);
+		const newMappings = new SvelteMap(mappings);
 		newMappings.set(recordedDeviceName, device);
 		mappings = newMappings;
 	}
@@ -95,7 +96,7 @@ const allDevicesMapped = $derived(
 		</Dialog.Header>
 
 		<div class="space-y-4 py-4">
-			{#each recordedDevices as recordedDevice}
+			{#each recordedDevices as recordedDevice (recordedDevice.name)}
 				{@const compatibleDevices = getCompatibleDevices(recordedDevice)}
 				{@const currentMapping = mappings.get(recordedDevice.name)}
 
@@ -106,7 +107,7 @@ const allDevicesMapped = $derived(
 							<h3 class="font-semibold text-card-foreground">{recordedDevice.name}</h3>
 						</div>
 						<div class="flex flex-wrap gap-1">
-							{#each recordedDevice.capabilities as capability}
+							{#each recordedDevice.capabilities as capability (capability)}
 								<span class="text-xs px-2 py-0.5 rounded-full bg-primary/10 text-primary border border-primary/20">
 									{capability}
 								</span>
@@ -129,7 +130,7 @@ const allDevicesMapped = $derived(
 								onchange={(e) => handleDeviceSelect(recordedDevice.name, e.currentTarget.value)}
 							>
 								<option value="" disabled>Select device...</option>
-								{#each compatibleDevices as device}
+								{#each compatibleDevices as device (device.name)}
 									<option value={device.id}>
 										{device.name}
 										{#if device.name === recordedDevice.name}(exact match){/if}

packages/frontend/src/routes/play/components/recording-save-dialog.svelte

@@ -96,7 +96,7 @@ function handleCancel() {
 			<!-- Device Info -->
 			<div class="space-y-2">
 				<Label>Devices Used</Label>
-				{#each deviceInfo as device}
+				{#each deviceInfo as device (device.name)}
 					<div
 						class="flex items-center gap-2 text-sm bg-muted/20 rounded px-3 py-2"
 					>