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/routes/me/+page.svelte

@@ -31,7 +31,6 @@ import {
 	FileDropZone,
 	MEGABYTE,
 } from "$lib/components/ui/file-drop-zone";
-import * as Avatar from "$lib/components/ui/avatar";
 import RecordingCard from "$lib/components/recording-card/recording-card.svelte";
 
 const { data } = $props();
@@ -175,7 +174,7 @@ async function handleDeleteRecording(id: string) {
 		await deleteRecording(id);
 		recordings = recordings.filter((r) => r.id !== id);
 		toast.success($_("me.recordings.delete_success"));
-	} catch (error) {
+	} catch {
 		toast.error($_("me.recordings.delete_error"));
 	}
 }
@@ -625,7 +624,7 @@ onMount(() => {
                                         </tr>
                                     </thead>
                                     <tbody>
-                                        {#each data.analytics.videos as video}
+                                        {#each data.analytics.videos as video (video.slug)}
                                             <tr class="border-b border-border/50 hover:bg-primary/5 transition-colors">
                                                 <td class="p-3">
                                                     <a href="/videos/{video.slug}" class="hover:text-primary transition-colors">