feat(frontend): add annotation visibility toggle to PDF toolbar

Eye/eye-slash button in the PDF controls bar lets the user hide all
annotation highlights to read the document unobstructed and show them again
with one click.

- Button only renders when at least one annotation exists
- Active state (hidden) highlighted with brand-mint/bg-white/10 so the
  current state is always clear
- i18n keys added for de/en/es

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -258,5 +258,7 @@
 	"doc_panel_annotate": "Annotieren",
 	"doc_panel_annotate_stop": "Fertig",
 	"doc_panel_annotation_thread_title": "Annotation",
-	"doc_panel_discussion_annotation_tab": "Annotation · Seite {page}"
+	"doc_panel_discussion_annotation_tab": "Annotation · Seite {page}",
+	"pdf_annotations_show": "Annotierungen anzeigen",
+	"pdf_annotations_hide": "Annotierungen verbergen"
 }

frontend/messages/en.json

@@ -258,5 +258,7 @@
 	"doc_panel_annotate": "Annotate",
 	"doc_panel_annotate_stop": "Done",
 	"doc_panel_annotation_thread_title": "Annotation",
-	"doc_panel_discussion_annotation_tab": "Annotation · Page {page}"
+	"doc_panel_discussion_annotation_tab": "Annotation · Page {page}",
+	"pdf_annotations_show": "Show annotations",
+	"pdf_annotations_hide": "Hide annotations"
 }

frontend/messages/es.json

@@ -258,5 +258,7 @@
 	"doc_panel_annotate": "Anotar",
 	"doc_panel_annotate_stop": "Listo",
 	"doc_panel_annotation_thread_title": "Anotación",
-	"doc_panel_discussion_annotation_tab": "Anotación · Página {page}"
+	"doc_panel_discussion_annotation_tab": "Anotación · Página {page}",
+	"pdf_annotations_show": "Mostrar anotaciones",
+	"pdf_annotations_hide": "Ocultar anotaciones"
 }

frontend/src/lib/components/PdfViewer.svelte

@@ -3,6 +3,7 @@ import { onMount } from 'svelte';
 import { SvelteMap } from 'svelte/reactivity';
 import type { PDFDocumentProxy, PDFPageProxy, RenderTask } from 'pdfjs-dist';
 import AnnotationLayer from './AnnotationLayer.svelte';
+import { m } from '$lib/paraglide/messages.js';
 
 let {
 	url,
@@ -55,6 +56,7 @@ type Annotation = {
 let annotations = $state<Annotation[]>([]);
 let annotateColor = $state('#ffff00');
 let commentCounts = new SvelteMap<string, number>();
+let showAnnotations = $state(true);
 
 onMount(async () => {
 	// Dynamic import keeps pdfjs out of the SSR bundle entirely
@@ -403,6 +405,53 @@ function zoomOut() {
 			{/if}
 		</div>
 
+		<!-- Annotation visibility toggle (shown when annotations exist) -->
+		{#if annotations.length > 0}
+			<button
+				onclick={() => (showAnnotations = !showAnnotations)}
+				aria-label={showAnnotations ? m.pdf_annotations_hide() : m.pdf_annotations_show()}
+				title={showAnnotations ? m.pdf_annotations_hide() : m.pdf_annotations_show()}
+				class="rounded p-1 transition {showAnnotations
+						? 'text-gray-300 hover:bg-white/10'
+						: 'bg-white/10 text-brand-mint'}"
+			>
+				{#if showAnnotations}
+					<svg
+						class="h-4 w-4"
+						viewBox="0 0 24 24"
+						fill="none"
+						stroke="currentColor"
+						stroke-width="2"
+					>
+						<path
+							stroke-linecap="round"
+							stroke-linejoin="round"
+							d="M15 12a3 3 0 11-6 0 3 3 0 016 0z"
+						/>
+						<path
+							stroke-linecap="round"
+							stroke-linejoin="round"
+							d="M2.458 12C3.732 7.943 7.523 5 12 5c4.478 0 8.268 2.943 9.542 7-1.274 4.057-5.064 7-9.542 7-4.477 0-8.268-2.943-9.542-7z"
+						/>
+					</svg>
+				{:else}
+					<svg
+						class="h-4 w-4"
+						viewBox="0 0 24 24"
+						fill="none"
+						stroke="currentColor"
+						stroke-width="2"
+					>
+						<path
+							stroke-linecap="round"
+							stroke-linejoin="round"
+							d="M13.875 18.825A10.05 10.05 0 0112 19c-4.478 0-8.268-2.943-9.543-7a9.97 9.97 0 011.563-3.029m5.858.908a3 3 0 114.243 4.243M9.878 9.878l4.242 4.242M9.88 9.88l-3.29-3.29m7.532 7.532l3.29 3.29M3 3l3.59 3.59m0 0A9.953 9.953 0 0112 5c4.478 0 8.268 2.943 9.543 7a10.025 10.025 0 01-4.132 5.411m0 0L21 21"
+						/>
+					</svg>
+				{/if}
+			</button>
+		{/if}
+
 		<!-- PDF canvas area -->
 		<div class="relative flex-1 overflow-auto">
 			{#if loading}
@@ -424,15 +473,17 @@ function zoomOut() {
 							class="textLayer"
 							style="position: absolute; top: 0; left: 0; overflow: hidden; pointer-events: none; line-height: 1;"
 						></div>
-						<AnnotationLayer
-							annotations={annotations.filter((a) => a.pageNumber === currentPage)}
-							canAnnotate={annotateMode}
-							color={annotateColor}
-							onDraw={handleAnnotationDraw}
-							onDelete={handleAnnotationDelete}
-							commentCounts={Object.fromEntries(commentCounts)}
-							onAnnotationClick={handleAnnotationClick}
-						/>
+						{#if showAnnotations}
+							<AnnotationLayer
+								annotations={annotations.filter((a) => a.pageNumber === currentPage)}
+								canAnnotate={annotateMode}
+								color={annotateColor}
+								onDraw={handleAnnotationDraw}
+								onDelete={handleAnnotationDelete}
+								commentCounts={Object.fromEntries(commentCounts)}
+								onAnnotationClick={handleAnnotationClick}
+							/>
+						{/if}
 					</div>
 				</div>
 			{/if}