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/Toaster.svelte

@@ -27,7 +27,9 @@
 		<button
 			type="button"
 			onclick={() => dismissToast(t.id)}
-			class="pointer-events-auto w-full max-w-sm rounded-xl px-4 py-3 text-left text-sm font-medium shadow-lg transition active:scale-[0.98] {toneClasses(t.tone)}"
+			class="pointer-events-auto w-full max-w-sm rounded-xl px-4 py-3 text-left text-sm font-medium shadow-lg transition active:scale-[0.98] {toneClasses(
+				t.tone
+			)}"
 			role={t.tone === 'error' || t.tone === 'warning' ? 'alert' : 'status'}
 			aria-live={t.tone === 'error' || t.tone === 'warning' ? 'assertive' : 'polite'}
 			data-testid="toast"