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/lib/components/header/header.svelte

@@ -1,12 +1,11 @@
 <script lang="ts">
 import { _ } from "svelte-i18n";
 import { page } from "$app/state";
-import PeonyIcon from "$lib/components/icon/peony-icon.svelte";
 import { Button } from "$lib/components/ui/button";
 import type { AuthStatus } from "$lib/types";
 import { logout } from "$lib/services";
 import { goto } from "$app/navigation";
-import { getAssetUrl, isModel } from "$lib/directus";
+import { getAssetUrl } from "$lib/directus";
 import LogoutButton from "../logout-button/logout-button.svelte";
 import Separator from "../ui/separator/separator.svelte";
 import { Avatar, AvatarFallback, AvatarImage } from "$lib/components/ui/avatar";
@@ -64,7 +63,7 @@ function isActiveLink(link: any) {
 
       <!-- Desktop Navigation -->
       <nav class="hidden w-full lg:flex items-center justify-center gap-8">
-        {#each navLinks as link}
+        {#each navLinks as link (link.href)}
           <a
             href={link.href}
             class={`text-sm hover:text-foreground transition-colors duration-200 font-medium relative group ${
@@ -219,7 +218,7 @@ function isActiveLink(link: any) {
               {$_('header.navigation')}
             </h3>
             <div class="grid gap-2">
-              {#each navLinks as link}
+              {#each navLinks as link (link.href)}
                 <a
                   href={link.href}
                   class="flex items-center justify-between rounded-xl border border-border/50 bg-card/50 p-4 backdrop-blur-sm transition-all hover:bg-card hover:border-primary/20 {isActiveLink(
@@ -259,7 +258,7 @@ function isActiveLink(link: any) {
                   onclick={closeMenu}
                 >
                   <div
-                    class={`flex h-10 w-10 items-center justify-center rounded-xl bg-gradient-to-br from-muted to-muted/50 transition-all group-hover:bg-card group-hover:from-primary/10 group-hover:to-accent/10`}
+                    class="flex h-10 w-10 items-center justify-center rounded-xl bg-gradient-to-br from-muted to-muted/50 transition-all group-hover:bg-card group-hover:from-primary/10 group-hover:to-accent/10"
                   >
                     <span
                       class="icon-[ri--dashboard-2-line] h-4 w-4 text-muted-foreground group-hover:text-foreground transition-colors"
@@ -286,7 +285,7 @@ function isActiveLink(link: any) {
                   onclick={closeMenu}
                 >
                   <div
-                    class={`flex h-10 w-10 items-center justify-center rounded-xl bg-gradient-to-br from-muted to-muted/50 transition-all group-hover:bg-card group-hover:from-primary/10 group-hover:to-accent/10`}
+                    class="flex h-10 w-10 items-center justify-center rounded-xl bg-gradient-to-br from-muted to-muted/50 transition-all group-hover:bg-card group-hover:from-primary/10 group-hover:to-accent/10"
                   >
                     <span
                       class="icon-[ri--rocket-line] h-4 w-4 text-muted-foreground group-hover:text-foreground transition-colors"
@@ -313,7 +312,7 @@ function isActiveLink(link: any) {
                   onclick={closeMenu}
                 >
                   <div
-                    class={`flex h-10 w-10 items-center justify-center rounded-xl bg-gradient-to-br from-muted to-muted/50 transition-all group-hover:bg-card group-hover:from-primary/10 group-hover:to-accent/10`}
+                    class="flex h-10 w-10 items-center justify-center rounded-xl bg-gradient-to-br from-muted to-muted/50 transition-all group-hover:bg-card group-hover:from-primary/10 group-hover:to-accent/10"
                   >
                     <span
                       class="icon-[ri--login-circle-line] h-4 w-4 text-muted-foreground group-hover:text-foreground transition-colors"
@@ -340,7 +339,7 @@ function isActiveLink(link: any) {
                   onclick={closeMenu}
                 >
                   <div
-                    class={`flex h-10 w-10 items-center justify-center rounded-xl bg-gradient-to-br from-muted to-muted/50 transition-all group-hover:bg-card group-hover:from-primary/10 group-hover:to-accent/10`}
+                    class="flex h-10 w-10 items-center justify-center rounded-xl bg-gradient-to-br from-muted to-muted/50 transition-all group-hover:bg-card group-hover:from-primary/10 group-hover:to-accent/10"
                   >
                     <span
                       class="icon-[ri--heart-add-2-line] h-4 w-4 text-muted-foreground group-hover:text-foreground transition-colors"