refactor: move document domain core to lib/document/

Moves ~25 components, utils (search, filename, groupDocuments,
documentStatusLabel, validateFile), bulkSelection store, and
TranscriptionSection sub-component. Fixes broken relative imports.

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

frontend/src/routes/documents/new/+page.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-import BulkDocumentEditLayout from '$lib/components/document/BulkDocumentEditLayout.svelte';
+import BulkDocumentEditLayout from '$lib/document/BulkDocumentEditLayout.svelte';
 
 let { data } = $props();
 </script>

frontend/src/routes/documents/new/FileSectionNew.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 import { m } from '$lib/paraglide/messages.js';
-import { parseFilename, stripExtension, type FilenameParseResult } from '$lib/utils/filename';
+import { parseFilename, stripExtension, type FilenameParseResult } from '$lib/document/filename';
 
 let {
 	onfileParsed