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/hooks.server.ts

@@ -2,12 +2,8 @@ import { isAuthenticated } from "$lib/services";
 import { logger, generateRequestId } from "$lib/logger";
 import type { Handle } from "@sveltejs/kit";
 
-// Log startup info once
-let hasLoggedStartup = false;
-if (!hasLoggedStartup) {
-	logger.startup();
-	hasLoggedStartup = true;
-}
+// Log startup info once (module-level code runs exactly once on import)
+logger.startup();
 
 export const handle: Handle = async ({ event, resolve }) => {
 	const { cookies, locals, url, request } = event;