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/ConfirmSheet.svelte

@@ -80,12 +80,16 @@
 			onclick={handleConfirm}
 			disabled={busy}
 			data-testid="confirm-sheet-confirm"
-			class="mb-3 flex w-full items-center justify-center gap-2 rounded-xl py-3 text-sm font-semibold text-white transition disabled:opacity-60 {tone === 'danger'
+			class="mb-3 flex w-full items-center justify-center gap-2 rounded-xl py-3 text-sm font-semibold text-white transition disabled:opacity-60 {tone ===
+			'danger'
 				? 'bg-red-600 hover:bg-red-700 active:bg-red-700 dark:bg-red-500 dark:hover:bg-red-400 dark:active:bg-red-400'
 				: 'bg-blue-600 hover:bg-blue-700 active:bg-blue-700 dark:bg-blue-500 dark:hover:bg-blue-400 dark:active:bg-blue-400'}"
 		>
 			{#if busy}
-				<span class="inline-block h-4 w-4 animate-spin rounded-full border-2 border-white/40 border-t-white" aria-hidden="true"></span>
+				<span
+					class="inline-block h-4 w-4 animate-spin rounded-full border-2 border-white/40 border-t-white"
+					aria-hidden="true"
+				></span>
 			{/if}
 			{confirmLabel}
 		</button>