feat(pdf): anchored scratch links replace board

Replace the rejected standalone sticky-card board with the real
feature: place a link anchor anywhere on a PDF page, tap it to open
split view whose right pane is THAT anchor's own infinite scratchpad
(keyed by anchor id) — like a paper sticky-note tab.

- ScratchLink model + scratch_links table (id, doc, page, nx, ny).
- PDF editor: "place link" tool drops/loads/shows tappable markers;
  tap opens SplitViewScreen for that anchor; long-press deletes.
- SplitViewScreen rebuilt on pdfrx (was syncfusion), right scratchpad
  keyed by scratchLinkId, new brush palette (was AnnotationToolbar).
- Remove board_screen + its test + the home board entry.

analyze clean, tests green.

lib/screens/home_screen.dart

@@ -11,10 +11,8 @@ import '../services/pdf_service.dart';
 import '../services/pptx_service.dart';
 import '../editor/canvas/pen_note_screen.dart';
 import '../editor/canvas/pen_slide_screen.dart';
-import 'board_screen.dart';
 import 'search_screen.dart';
 import 'settings_screen.dart';
-import 'split_view_screen.dart';
 
 // [M1] Relative date helper — no new package dependencies.
 String _formatDate(DateTime d) {
@@ -61,15 +59,6 @@ class HomeScreen extends ConsumerWidget {
             tooltip: l.importPpt,
             onPressed: () => _importPptx(context),
           ),
-          IconButton(
-            icon: const Icon(Icons.dashboard_customize),
-            tooltip: l.boardOpen,
-            onPressed: () {
-              Navigator.of(
-                context,
-              ).push(MaterialPageRoute(builder: (_) => const BoardScreen()));
-            },
-          ),
           IconButton(
             icon: const Icon(Icons.search),
             tooltip: l.search,
@@ -502,32 +491,18 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
             '${document.docType.toUpperCase()} · ${document.pageCount} pages · $dateStr',
             style: Theme.of(context).textTheme.bodySmall,
           ),
-          // [H2] Trailing row: split-view (PDF only) + remove
-          trailing: Row(
-            mainAxisSize: MainAxisSize.min,
-            children: [
-              if (isPdf)
-                IconButton(
-                  icon: Icon(
-                    Icons.vertical_split,
-                    color: _hovering
-                        ? Theme.of(context).colorScheme.primary
-                        : subtleColor.withValues(alpha: 0.4),
-                  ),
-                  tooltip: 'Open in Split View',
-                  onPressed: () => _openSplitView(context),
-                ),
-              IconButton(
-                icon: Icon(
-                  Icons.delete_outline,
-                  color: _hovering
-                      ? Theme.of(context).colorScheme.error
-                      : subtleColor.withValues(alpha: 0.4),
-                ),
-                tooltip: 'Remove document',
-                onPressed: () => _confirmDelete(context),
-              ),
-            ],
+          // [H2] Trailing remove button. Split view is now reached only by
+          // tapping a scratch-link anchor inside the PDF editor, so the
+          // standalone split-view entry was removed.
+          trailing: IconButton(
+            icon: Icon(
+              Icons.delete_outline,
+              color: _hovering
+                  ? Theme.of(context).colorScheme.error
+                  : subtleColor.withValues(alpha: 0.4),
+            ),
+            tooltip: 'Remove document',
+            onPressed: () => _confirmDelete(context),
           ),
           // [L2] Routing bug fix: route by docType
           onTap: () => _openDocument(context),
@@ -577,19 +552,7 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
     }
   }
 
-  void _openSplitView(BuildContext context) {
-    Navigator.of(context).push(
-      MaterialPageRoute(
-        builder: (_) => SplitViewScreen(
-          filePath: widget.document.filePath,
-          documentId: widget.document.id,
-        ),
-      ),
-    );
-  }
-
   void _showContextMenu(BuildContext context, Offset position) async {
-    final isPdf = widget.document.docType == 'pdf';
     final result = await showMenu<String>(
       context: context,
       position: RelativeRect.fromLTRB(
@@ -609,17 +572,6 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
             ],
           ),
         ),
-        if (isPdf)
-          PopupMenuItem(
-            value: 'split',
-            child: Row(
-              children: const [
-                Icon(Icons.vertical_split),
-                SizedBox(width: 8),
-                Text('Open in Split View'),
-              ],
-            ),
-          ),
         PopupMenuItem(
           value: 'remove',
           child: Row(
@@ -641,15 +593,12 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
     if (!mounted) return;
     if (result == 'open') {
       _openDocument(this.context);
-    } else if (result == 'split') {
-      _openSplitView(this.context);
     } else if (result == 'remove') {
       _confirmDelete(this.context);
     }
   }
 
   void _showDocumentMenu(BuildContext context) {
-    final isPdf = widget.document.docType == 'pdf';
     showModalBottomSheet(
       context: context,
       builder: (ctx) {
@@ -657,16 +606,6 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
           child: Column(
             mainAxisSize: MainAxisSize.min,
             children: [
-              if (isPdf)
-                ListTile(
-                  leading: const Icon(Icons.vertical_split),
-                  title: const Text('Open in Split View'),
-                  subtitle: const Text('PDF reference + scratchpad'),
-                  onTap: () {
-                    Navigator.of(ctx).pop();
-                    _openSplitView(context);
-                  },
-                ),
               ListTile(
                 leading: const Icon(Icons.delete_outline, color: Colors.red),
                 title: const Text(