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/signup/+page.svelte

@@ -14,7 +14,6 @@ import { Label } from "$lib/components/ui/label";
 import { Checkbox } from "$lib/components/ui/checkbox";
 import { toast } from "svelte-sonner";
 import * as Alert from "$lib/components/ui/alert";
-import PeonyIcon from "$lib/components/icon/peony-icon.svelte";
 import { register } from "$lib/services";
 import PeonyBackground from "$lib/components/background/peony-background.svelte";
 import Meta from "$lib/components/meta/meta.svelte";