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/api.ts

@@ -15,11 +15,7 @@ export class ApiError extends Error {
 
 const TIMEOUT_MS = 20_000;
 
-async function request<T>(
-	method: string,
-	path: string,
-	body?: unknown
-): Promise<T> {
+async function request<T>(method: string, path: string, body?: unknown): Promise<T> {
 	const headers: Record<string, string> = {};
 	const token = getToken();
 	if (token) {
@@ -75,7 +71,11 @@ async function request<T>(
 			clearAuth();
 		}
 		const d = (data ?? {}) as { error?: string; message?: string };
-		throw new ApiError(res.status, d.error ?? 'unknown', d.message ?? `Serverfehler (${res.status}).`);
+		throw new ApiError(
+			res.status,
+			d.error ?? 'unknown',
+			d.message ?? `Serverfehler (${res.status}).`
+		);
 	}
 
 	return data as T;