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/routes/documents/[id]/+page.svelte

@@ -4,8 +4,7 @@ import DocumentTopBar from '$lib/components/DocumentTopBar.svelte';
 import DocumentViewer from '$lib/components/DocumentViewer.svelte';
 import DocumentBottomPanel from '$lib/components/DocumentBottomPanel.svelte';
 import AnnotationSidePanel from '$lib/components/AnnotationSidePanel.svelte';
-
-type Tab = 'metadata' | 'transcription' | 'discussion' | 'history';
+import type { DocumentPanelTab } from '$lib/types';
 
 let { data } = $props();
 
@@ -72,7 +71,7 @@ const LS_KEY_TAB = 'doc-panel-tab';
 let panelOpen = $state(false);
 let panelHeight = $state(0); // set to full height on mount
 let navHeight = $state(0);
-let activeTab = $state<Tab>('metadata');
+let activeTab = $state<DocumentPanelTab>('metadata');
 let localStorageRestored = $state(false);
 
 onMount(() => {
@@ -82,7 +81,7 @@ onMount(() => {
 	const savedTab = localStorage.getItem(LS_KEY_TAB);
 
 	if (savedTab && ['metadata', 'transcription', 'discussion', 'history'].includes(savedTab)) {
-		activeTab = savedTab as Tab;
+		activeTab = savedTab as DocumentPanelTab;
 	}
 	const topbar = document.querySelector('[data-topbar]');
 	panelHeight = window.innerHeight - navHeight - (topbar?.getBoundingClientRect().height ?? 0);