refactor: move document transcription, annotation, viewer sub-packages

- transcription/: TranscriptionBlock, Column, EditView, PanelHeader, ReadView,
  Section + transcriptionMarkers, blockConflictMerge, saveBlockWithConflictRetry
  + useBlockAutoSave, useBlockDragDrop hooks
- annotation/: AnnotationLayer, AnnotationShape, AnnotationEditOverlay
- viewer/: PdfViewer, PdfControls + useFileLoader, usePdfRenderer hooks

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

frontend/src/lib/document/transcription/blockConflictMerge.ts

@@ -0,0 +1,50 @@
+import type { PersonMention, TranscriptionBlockData } from '$lib/types';
+
+/**
+ * Sentinel thrown by saveBlockWithConflictRetry after a 409 rename-mid-edit
+ * has been merged into local state. Surfaces to the autosave hook as an
+ * error (so the UI shows the retry indicator), but distinguishable from a
+ * genuine network failure via the code. Carries the merged block snapshot
+ * on its `merged` property so the caller can update local state without
+ * a second roundtrip.
+ */
+export class BlockConflictResolvedError extends Error {
+	readonly code = 'CONFLICT_RESOLVED' as const;
+	merged?: TranscriptionBlockData;
+	constructor(blockId: string) {
+		super(
+			`Block ${blockId} was rebased onto the latest server snapshot — retry to save the merged result`
+		);
+		this.name = 'BlockConflictResolvedError';
+	}
+}
+
+type MergeArgs = {
+	serverBlock: TranscriptionBlockData;
+	localText: string;
+	localMentions: PersonMention[];
+};
+
+/**
+ * Resolves a 409-Conflict from the server by combining the latest server
+ * snapshot with the transcriber's unsaved local edits (B12b).
+ *
+ * Rules:
+ * - The transcriber's typed text always wins — never overwrite their input.
+ * - Server is the source of truth for the displayName of any person it
+ *   knows about; renames that just landed on the server replace stale local
+ *   names by personId.
+ * - Local-only mentions added since the last save are preserved.
+ * - All non-mention fields (version, sortOrder, reviewed, updatedAt, ...)
+ *   come from the server snapshot so the next save sends the current
+ *   revision and matches the latest persisted state.
+ */
+export function mergeBlockOnConflict(args: MergeArgs): TranscriptionBlockData {
+	const serverIds = new Set(args.serverBlock.mentionedPersons.map((m) => m.personId));
+	const localOnly = args.localMentions.filter((m) => !serverIds.has(m.personId));
+	return {
+		...args.serverBlock,
+		text: args.localText,
+		mentionedPersons: [...args.serverBlock.mentionedPersons, ...localOnly]
+	};
+}