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/eslint.config.js

@@ -0,0 +1,70 @@
+import js from '@eslint/js';
+import ts from 'typescript-eslint';
+import svelte from 'eslint-plugin-svelte';
+import prettier from 'eslint-config-prettier';
+import globals from 'globals';
+import svelteConfig from './svelte.config.js';
+
+/**
+ * Flat-config ESLint for the SvelteKit frontend. Type-aware where it's cheap; Prettier owns
+ * formatting (its config is last so it disables every stylistic rule that would fight the formatter).
+ */
+export default ts.config(
+	js.configs.recommended,
+	...ts.configs.recommended,
+	...svelte.configs.recommended,
+	prettier,
+	...svelte.configs.prettier,
+	{
+		languageOptions: {
+			globals: { ...globals.browser, ...globals.node }
+		}
+	},
+	{
+		files: ['**/*.svelte', '**/*.svelte.ts', '**/*.svelte.js'],
+		languageOptions: {
+			parserOptions: {
+				projectService: true,
+				extraFileExtensions: ['.svelte'],
+				parser: ts.parser,
+				svelteConfig
+			}
+		}
+	},
+	{
+		rules: {
+			// A leading underscore is the deliberate "intentionally unused" marker (e.g. `{#each x as _}`).
+			'@typescript-eslint/no-unused-vars': [
+				'error',
+				{ argsIgnorePattern: '^_', varsIgnorePattern: '^_', caughtErrorsIgnorePattern: '^_' }
+			],
+			// Off on purpose. This rule wants every `goto('/x')` / `href="/x"` wrapped in `resolve()`
+			// for typed routes. That is a taste/ergonomics preference, not a correctness rule — string
+			// routes work fine and the app uses them deliberately. We keep the rules that catch actual
+			// bugs (require-each-key, prefer-svelte-reactivity) and drop this churn.
+			'svelte/no-navigation-without-resolve': 'off',
+			// Off: `svelte-ignore` comments are consumed by the Svelte compiler / svelte-check, which
+			// ESLint cannot see — so it reports every one as "unused" even when it is actively
+			// suppressing a real svelte-check a11y warning. Removing them on ESLint's say-so would
+			// reintroduce those warnings. svelte-check is the authority on these, not ESLint.
+			'svelte/no-unused-svelte-ignore': 'off'
+		}
+	},
+	{
+		// Test fixtures legitimately use `any` for partial/mock shapes (e.g. a stub upload that only
+		// sets the two fields the function under test reads). Not worth threading full types through.
+		files: ['**/*.test.ts'],
+		rules: { '@typescript-eslint/no-explicit-any': 'off' }
+	},
+	{
+		ignores: [
+			'.svelte-kit/',
+			'build/',
+			'dist/',
+			'node_modules/',
+			'static/export-viewer/',
+			'*.config.js',
+			'*.config.ts'
+		]
+	}
+);