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/backend/src/index.ts

@@ -1,4 +1,4 @@
-import Fastify, { FastifyRequest, FastifyReply } from "fastify";
+import Fastify, { type FastifyRequest, type FastifyReply } from "fastify";
 import fastifyCookie from "@fastify/cookie";
 import fastifyCors from "@fastify/cors";
 import fastifyMultipart from "@fastify/multipart";