chore(frontend): add ESLint + Prettier; fix real findings; format the tree

The frontend had no JS/TS linter — only svelte-check. Add flat-config ESLint (typescript-eslint
+ eslint-plugin-svelte) and Prettier (tabs/single-quote, matching the existing style).

Rules encode "catch bugs, not enforce taste":
  - svelte/require-each-key KEPT — it is the exact bug class as the feed mis-tap fix. Fixed every
    flagged block: keyed activeFilters, filteredUsers, stagedFiles (by previewUrl), captionTags,
    admin tabs/jobs/users, the export-viewer suggestions/filters/comments, and the static skeleton
    loops.
  - svelte/prefer-svelte-reactivity KEPT — inline-disabled only the verified-safe sites (a local
    freq Map in a $derived.by, throwaway URLSearchParams query builders), with a reason each.
  - svelte/no-navigation-without-resolve OFF — wants resolve() around every goto()/href; taste, not
    a bug, and pure churn.
  - svelte/no-unused-svelte-ignore OFF — those comments are consumed by svelte-check, which ESLint
    can't see, so it wrongly calls them unused; removing them would reintroduce a11y warnings.
  - no-explicit-any OFF for *.test.ts only (partial fixtures legitimately use any).

Real code fixes beyond keys: removed a dead jobLabel(), an unused ViewerComment import and unused
catch binding, an unused scroll-lock arg, and replaced an empty interface with a type alias.

Then `prettier --write` (54 files). Formatting only. Verified: eslint clean, svelte-check 0 errors,
vitest 46 passed, vite build succeeds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/upload-queue.ts

@@ -97,7 +97,9 @@ async function requeueRetriable(): Promise<void> {
 	}
 	queueItems.update((items) =>
 		items.map((item) =>
-			item.status === 'error' ? { ...item, status: 'pending' as const, progress: 0, error: undefined } : item
+			item.status === 'error'
+				? { ...item, status: 'pending' as const, progress: 0, error: undefined }
+				: item
 		)
 	);
 }
@@ -637,11 +639,7 @@ async function uploadItem(id: string): Promise<void> {
 	}
 }
 
-function updateItemStatus(
-	id: string,
-	status: QueueItem['status'],
-	error?: string
-): void {
+function updateItemStatus(id: string, status: QueueItem['status'], error?: string): void {
 	queueItems.update((items) =>
 		items.map((item) =>
 			item.id === id