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/about/+page.svelte

@@ -108,7 +108,7 @@ const values = [
     <section class="py-16 bg-card/30">
         <div class="container mx-auto px-4">
             <div class="grid grid-cols-2 md:grid-cols-4 gap-8">
-                {#each stats as stat}
+                {#each stats as stat (stat.icon)}
                     <div class="text-center">
                         <div
                             class="w-16 h-16 bg-gradient-to-br from-primary/20 to-accent/20 rounded-full flex items-center justify-center mx-auto mb-4"
@@ -176,7 +176,7 @@ const values = [
             </div>
 
             <div class="grid grid-cols-1 md:grid-cols-2 gap-8 max-w-4xl mx-auto">
-                {#each values as value}
+                {#each values as value (value.title)}
                     <Card
                         class="bg-gradient-to-br from-card to-card/50 border-primary/20 hover:shadow-xl hover:shadow-primary/10 transition-all duration-300"
                     >
@@ -214,7 +214,7 @@ const values = [
             </div>
 
             <div class="grid grid-cols-1 md:grid-cols-2 gap-8">
-                {#each team as member}
+                {#each team as member (member.name)}
                     <Card
                         class="bg-gradient-to-br from-card to-card/50 border-primary/20 hover:shadow-xl hover:shadow-primary/10 transition-all duration-300 hover:-translate-y-2"
                     >