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/login/+page.svelte

@@ -10,8 +10,6 @@ import {
 } from "$lib/components/ui/card";
 import { Input } from "$lib/components/ui/input";
 import { Label } from "$lib/components/ui/label";
-import { Checkbox } from "$lib/components/ui/checkbox";
-import PeonyIcon from "$lib/components/icon/peony-icon.svelte";
 import * as Alert from "$lib/components/ui/alert";
 import { goto } from "$app/navigation";
 import { login } from "$lib/services";
@@ -24,7 +22,7 @@ let email = $state("");
 let password = $state("");
 let error = $state("");
 let showPassword = $state(false);
-let rememberMe = $state(false);
+let _rememberMe = $state(false);
 let isLoading = $state(false);
 let isError = $state(false);
 
@@ -212,7 +210,7 @@ onMount(() => {
                 <!-- Sign Up Link -->
                 <div class="text-center">
                     <p class="text-sm text-muted-foreground">
-                        {$_("auth.login.no_account")}{" "}
+                        {$_("auth.login.no_account")}
                         <a href="/signup" class="text-primary hover:underline font-medium"
                             >{$_("auth.login.sign_up_link")}</a
                         >