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)}