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/device-card/device-card.svelte

@@ -56,7 +56,7 @@ function isActive() {
                 </div>
                 <div>
                     <h3
-                        class={`font-semibold text-card-foreground group-hover:text-primary transition-colors`}
+                        class="font-semibold text-card-foreground group-hover:text-primary transition-colors"
                     >
                         {device.name}
                     </h3>
@@ -139,7 +139,7 @@ function isActive() {
     </div> -->
 
         <!-- Action Button -->
-        {#each device.actuators as actuator, idx}
+        {#each device.actuators as actuator, idx (idx)}
             <div class="space-y-2">
                 <Label for={`device-${device.info.index}-${actuator.featureIndex}-${actuator.outputType}`}
                     >{$_(