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/DocumentBottomPanel.svelte

@@ -4,27 +4,7 @@ import PanelMetadata from './PanelMetadata.svelte';
 import PanelTranscription from './PanelTranscription.svelte';
 import PanelDiscussion from './PanelDiscussion.svelte';
 import PanelHistory from './PanelHistory.svelte';
-
-type Tab = 'metadata' | 'transcription' | 'discussion' | 'history';
-
-type CommentReply = {
-	id: string;
-	authorId: string | null;
-	authorName: string;
-	content: string;
-	createdAt: string;
-	updatedAt: string;
-};
-
-type Comment = {
-	id: string;
-	authorId: string | null;
-	authorName: string;
-	content: string;
-	createdAt: string;
-	updatedAt: string;
-	replies: CommentReply[];
-};
+import type { Comment, DocumentPanelTab } from '$lib/types';
 
 type Doc = {
 	id: string;
@@ -47,7 +27,7 @@ type Props = {
 	canAdmin: boolean;
 	open: boolean;
 	height: number;
-	activeTab: Tab;
+	activeTab: DocumentPanelTab;
 };
 
 let {
@@ -72,7 +52,7 @@ function fullHeight() {
 	return window.innerHeight - (topbar?.getBoundingClientRect().bottom ?? 0);
 }
 
-function openTab(tab: Tab) {
+function openTab(tab: DocumentPanelTab) {
 	activeTab = tab;
 	if (!open) {
 		open = true;
@@ -110,7 +90,7 @@ function onDragEnd() {
 	isDragging = false;
 }
 
-const tabs: { id: Tab; label: () => string }[] = [
+const tabs: { id: DocumentPanelTab; label: () => string }[] = [
 	{ id: 'metadata', label: m.doc_panel_tab_metadata },
 	{ id: 'transcription', label: m.doc_panel_tab_transcription },
 	{ id: 'discussion', label: m.doc_panel_tab_discussion },