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>

eslint.config.js

@@ -0,0 +1,59 @@
+import js from "@eslint/js";
+import ts from "typescript-eslint";
+import svelte from "eslint-plugin-svelte";
+import prettier from "eslint-config-prettier";
+import globals from "globals";
+
+export default ts.config(
+  js.configs.recommended,
+  ...ts.configs.recommended,
+  ...svelte.configs["flat/recommended"],
+  prettier,
+  ...svelte.configs["flat/prettier"],
+  {
+    languageOptions: {
+      globals: {
+        ...globals.browser,
+        ...globals.node,
+      },
+    },
+  },
+  {
+    files: ["**/*.svelte"],
+    languageOptions: {
+      parserOptions: {
+        parser: ts.parser,
+      },
+    },
+  },
+  {
+    rules: {
+      // Allow unused vars prefixed with _ (common pattern for intentional ignores)
+      "@typescript-eslint/no-unused-vars": [
+        "error",
+        { argsIgnorePattern: "^_", varsIgnorePattern: "^_" },
+      ],
+      // Allow explicit any sparingly — we're adults here
+      "@typescript-eslint/no-explicit-any": "warn",
+      // Enforce consistent type imports
+      "@typescript-eslint/consistent-type-imports": [
+        "error",
+        { prefer: "type-imports", fixStyle: "inline-type-imports" },
+      ],
+      // This rule is meant for onNavigate() callbacks only; standard SvelteKit href/goto is fine
+      "svelte/no-navigation-without-resolve": "off",
+      // {@html} is used intentionally for trusted content (e.g. legal page)
+      "svelte/no-at-html-tags": "warn",
+    },
+  },
+  {
+    ignores: [
+      "**/build/",
+      "**/.svelte-kit/",
+      "**/dist/",
+      "**/node_modules/",
+      "**/migrations/",
+      "packages/buttplug/**",
+    ],
+  },
+);