refactor(types): extract shared types to \$lib/types.ts

Eliminates type duplication across 6 files by introducing a single
shared types module:

- Comment + CommentReply: were identically defined in CommentThread,
  PanelDiscussion, and DocumentBottomPanel
- DocumentPanelTab: was identically defined in DocumentBottomPanel
  and documents/[id]/+page.svelte
- Annotation: was defined in both AnnotationLayer and PdfViewer
  (PdfViewer's variant with fileHash? is now the canonical definition)

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

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 type { Annotation } from '$lib/types';
 import { m } from '$lib/paraglide/messages.js';
 
 let {
@@ -43,19 +44,6 @@ let textLayerInstance: { cancel: () => void } | null = null;
 let pdfjsLib: typeof import('pdfjs-dist') | null = null;
 let pdfjsReady = $state(false);
 
-type Annotation = {
-	id: string;
-	documentId: string;
-	pageNumber: number;
-	x: number;
-	y: number;
-	width: number;
-	height: number;
-	color: string;
-	createdAt: string;
-	fileHash?: string | null;
-};
-
 let annotations = $state<Annotation[]>([]);
 let annotateColor = $state('#ffff00');
 let commentCounts = new SvelteMap<string, number>();