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/components/Modal.svelte

@@ -19,14 +19,7 @@
 		children: Snippet;
 	}
 
-	let {
-		open,
-		titleId,
-		ariaLabel,
-		onClose,
-		closeOnBackdrop = true,
-		children
-	}: Props = $props();
+	let { open, titleId, ariaLabel, onClose, closeOnBackdrop = true, children }: Props = $props();
 
 	$effect(() => {
 		if (open && !titleId && !ariaLabel && typeof console !== 'undefined') {
@@ -56,7 +49,9 @@
 			use:focusTrap={{ onclose: onClose }}
 			use:scrollLock
 		>
-			<div class="pointer-events-auto w-full max-w-sm rounded-2xl bg-white p-6 shadow-xl dark:bg-gray-900">
+			<div
+				class="pointer-events-auto w-full max-w-sm rounded-2xl bg-white p-6 shadow-xl dark:bg-gray-900"
+			>
 				{@render children()}
 			</div>
 		</div>