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/AnnotationLayer.svelte

@@ -1,15 +1,5 @@
 <script lang="ts">
-type Annotation = {
-	id: string;
-	documentId: string;
-	pageNumber: number;
-	x: number;
-	y: number;
-	width: number;
-	height: number;
-	color: string;
-	createdAt: string;
-};
+import type { Annotation } from '$lib/types';
 
 type DrawRect = {
 	x: number;