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/sharing-popup/share-button.svelte

@@ -1,6 +1,4 @@
 <script lang="ts">
-import { Button } from "$lib/components/ui/button";
-
 interface Props {
 	onclick: () => void;
 	icon: string;

packages/frontend/src/lib/components/sharing-popup/share-services.svelte

@@ -50,7 +50,7 @@ const copyLink = async () => {
 	try {
 		await navigator.clipboard.writeText(content.url);
 		toast.success($_("sharing_popup.success.copy"));
-	} catch (err) {
+	} catch {
 		// Fallback for older browsers
 		const textArea = document.createElement("textarea");
 		textArea.value = content.url;