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>

prettier.config.js

@@ -0,0 +1,18 @@
+/** @type {import('prettier').Config} */
+export default {
+  useTabs: false,
+  tabWidth: 2,
+  singleQuote: false,
+  trailingComma: "all",
+  printWidth: 100,
+  semi: true,
+  plugins: ["prettier-plugin-svelte"],
+  overrides: [
+    {
+      files: "*.svelte",
+      options: {
+        parser: "svelte",
+      },
+    },
+  ],
+};