sync: Major UI updates from private repo (Jan 16-24)

Features synced:
- UI Panels Refactor: flexible pane system with ChatPane, NodePane,
  DimensionsPane, WorkflowsPane, ViewsPane, MapPane
- Source Content Reader: content type detection + 4 formatters
  (transcript, book, markdown, raw)
- Source Content Search: Cmd+F search in Source Reader
- Feed Layout Overhaul: LeftToolbar, SplitHandle, CollapsedRail
- Map Panel: promoted to first-class pane
- Edge policy simplification: auto-inference + direction correction

New files:
- src/components/panes/* (pane system)
- src/components/layout/CollapsedRail.tsx
- src/components/layout/LeftToolbar.tsx
- src/components/layout/SplitHandle.tsx
- src/components/focus/source/* (Source Reader + formatters)
- src/components/views/ViewsOverlay.tsx

Updated:
- ThreePanelLayout.tsx (complete refactor)
- FocusPanel.tsx (Source tab integration)
- API routes (dimensions GET, edges auto-inference)
- Database services (nodes, edges, dimensions)

Dependencies added:
- react-markdown
- remark-gfm

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/edges/route.ts

@@ -27,10 +27,10 @@ export async function POST(request: NextRequest) {
     const body = await request.json();
     
     // Validate required fields
-    if (!body.from_node_id || !body.to_node_id || typeof body.explanation !== 'string') {
+    if (!body.from_node_id || !body.to_node_id) {
       return NextResponse.json({
         success: false,
-        error: 'Missing required fields: from_node_id, to_node_id, and explanation are required'
+        error: 'Missing required fields: from_node_id and to_node_id are required'
       }, { status: 400 });
     }
 
@@ -57,15 +57,9 @@ export async function POST(request: NextRequest) {
 
     const fromId = parseInt(body.from_node_id);
     const toId = parseInt(body.to_node_id);
+    // Explanation can be empty - service will auto-generate
     const explanation = String(body.explanation || '').trim();
 
-    if (!explanation) {
-      return NextResponse.json({
-        success: false,
-        error: 'explanation is required and cannot be empty'
-      }, { status: 400 });
-    }
-
     const skipInference = Boolean(body.skip_inference);
     const createdVia = (() => {
       const raw = typeof body.created_via === 'string' ? body.created_via : '';