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/HeartBurst.svelte

@@ -5,15 +5,6 @@
 	let { active }: Props = $props();
 </script>
 
-<style>
-	@keyframes heart-burst {
-		0% { transform: scale(0.5); opacity: 0; }
-		30% { transform: scale(1.2); opacity: 1; }
-		70% { transform: scale(1); opacity: 1; }
-		100% { transform: scale(1.4); opacity: 0; }
-	}
-</style>
-
 {#if active}
 	<span
 		class="pointer-events-none absolute inset-0 flex items-center justify-center"
@@ -26,7 +17,30 @@
 			fill="currentColor"
 			viewBox="0 0 24 24"
 		>
-			<path d="M12 21s-7-4.534-9.5-9.034C.5 9.466 2.5 5 7 5c2.09 0 3.534 1.083 5 3 1.466-1.917 2.91-3 5-3 4.5 0 6.5 4.466 4.5 6.966C19 16.466 12 21 12 21z" />
+			<path
+				d="M12 21s-7-4.534-9.5-9.034C.5 9.466 2.5 5 7 5c2.09 0 3.534 1.083 5 3 1.466-1.917 2.91-3 5-3 4.5 0 6.5 4.466 4.5 6.966C19 16.466 12 21 12 21z"
+			/>
 		</svg>
 	</span>
 {/if}
+
+<style>
+	@keyframes heart-burst {
+		0% {
+			transform: scale(0.5);
+			opacity: 0;
+		}
+		30% {
+			transform: scale(1.2);
+			opacity: 1;
+		}
+		70% {
+			transform: scale(1);
+			opacity: 1;
+		}
+		100% {
+			transform: scale(1.4);
+			opacity: 0;
+		}
+	}
+</style>