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/package.json

@@ -8,7 +8,8 @@
     "dev": "vite",
     "build": "vite build",
     "preview": "vite preview",
-    "start": "node ./build"
+    "start": "node ./build",
+    "check": "svelte-check --tsconfig ./tsconfig.json --threshold warning"
   },
   "devDependencies": {
     "@iconify-json/ri": "^1.2.10",
@@ -30,6 +31,7 @@
     "prettier-plugin-svelte": "^3.5.1",
     "super-sitemap": "^1.0.7",
     "svelte": "^5.53.7",
+    "svelte-check": "^4.4.4",
     "svelte-sonner": "^1.0.8",
     "tailwind-merge": "^3.5.0",
     "tailwind-variants": "^3.2.2",