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/faq/+page.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 import { _ } from "svelte-i18n";
+import { SvelteSet } from "svelte/reactivity";
 import {
 	Card,
 	CardContent,
@@ -12,7 +13,7 @@ import PeonyBackground from "$lib/components/background/peony-background.svelte"
 import Meta from "$lib/components/meta/meta.svelte";
 
 let searchQuery = $state("");
-let expandedItems = $state<Set<number>>(new Set());
+let expandedItems = new SvelteSet<number>();
 
 const faqCategories = [
 	{
@@ -171,7 +172,7 @@ const filteredQuestions = $derived(() => {
 });
 
 function toggleExpanded(id: number) {
-	const newExpanded = new Set(expandedItems);
+	const newExpanded = new SvelteSet(expandedItems);
 	if (newExpanded.has(id)) {
 		newExpanded.delete(id);
 	} else {
@@ -224,7 +225,7 @@ function toggleExpanded(id: number) {
                     })}
                 </h2>
                 <div class="space-y-4">
-                    {#each filteredQuestions() as question}
+                    {#each filteredQuestions() as question (question.id)}
                         <Card
                             class="bg-gradient-to-br from-card/90 via-card/95 to-card/85 backdrop-blur-xl shadow-lg shadow-primary/10"
                         >
@@ -273,7 +274,7 @@ function toggleExpanded(id: number) {
             <!-- Category View -->
             <div class="max-w-6xl mx-auto">
                 <div class="grid grid-cols-1 lg:grid-cols-2 gap-8">
-                    {#each faqCategories as category}
+                    {#each faqCategories as category (category.id)}
                         <Card
                             class="bg-gradient-to-br from-card/90 via-card/95 to-card/85 backdrop-blur-xl shadow-lg shadow-primary/10"
                         >
@@ -290,7 +291,7 @@ function toggleExpanded(id: number) {
                             </CardHeader>
                             <CardContent class="pt-0">
                                 <div class="space-y-3">
-                                    {#each category.questions as question}
+                                    {#each category.questions as question (question.id)}
                                         <div
                                             class="border border-border/50 rounded-lg overflow-hidden"
                                         >