fix(frontend): resolve svelte-check type errors

- vite-mock-plugin: define the missing MockFile type and annotate the
  MOCK_FILES / MOCK_TRASH arrays so restore (unshift) type-checks.
- categories/[id], tags/[id]: page.params.id is string | undefined under
  noUncheckedIndexedAccess — guard loadTags and default the TagRuleEditor
  tagId so the routes type-check.

svelte-check now reports 0 errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/categories/[id]/+page.svelte

@@ -48,7 +48,8 @@
 		void loadTags(id, 0);
 	});
 
-	async function loadTags(id: string, startOffset: number) {
+	async function loadTags(id: string | undefined, startOffset: number) {
+		if (!id) return;
 		tagsLoading = true;
 		try {
 			const params = new URLSearchParams({

frontend/src/routes/tags/[id]/+page.svelte

@@ -172,7 +172,7 @@
 			<!-- Tag rules -->
 			<section class="section">
 				<h2 class="section-title">Implied tags</h2>
-				<TagRuleEditor {tagId} {rules} onRulesChange={(r) => (rules = r)} />
+				<TagRuleEditor tagId={tagId ?? ''} {rules} onRulesChange={(r) => (rules = r)} />
 			</section>
 
 		{/if}