fix: accept WebSocket before validation to prevent opaque 403 errors

All 5 WebSocket endpoints (expand, spec, assistant, terminal, project)
were closing the connection before calling accept() when validation
failed. Starlette converts pre-accept close into an HTTP 403, giving
clients no meaningful error information.

Server changes:
- Move websocket.accept() before all validation checks in every WS handler
- Send JSON error message before closing so clients get actionable errors
- Fix validate_project_name usage (raises HTTPException, not returns bool)
- ConnectionManager.connect() no longer calls accept() (caller's job)

Client changes:
- All 3 WS hooks (useWebSocket, useExpandChat, useSpecChat) skip
  reconnection on 4xxx close codes (application errors won't self-resolve)
- Gate expand button, keyboard shortcut, and modal on hasSpec
- Add hasSpec to useEffect dependency array to prevent stale closure
- Update keyboard shortcuts help text for E key context

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

ui/src/components/KanbanBoard.tsx

@@ -51,7 +51,7 @@ export function KanbanBoard({ features, onFeatureClick, onAddFeature, onExpandPr
         onFeatureClick={onFeatureClick}
         onAddFeature={onAddFeature}
         onExpandProject={onExpandProject}
-        showExpandButton={hasFeatures}
+        showExpandButton={hasFeatures && hasSpec}
         onCreateSpec={onCreateSpec}
         showCreateSpec={!hasSpec && !hasFeatures}
       />

ui/src/components/KeyboardShortcutsHelp.tsx

@@ -19,7 +19,7 @@ const shortcuts: Shortcut[] = [
   { key: 'D', description: 'Toggle debug panel' },
   { key: 'T', description: 'Toggle terminal tab' },
   { key: 'N', description: 'Add new feature', context: 'with project' },
-  { key: 'E', description: 'Expand project with AI', context: 'with features' },
+  { key: 'E', description: 'Expand project with AI', context: 'with spec & features' },
   { key: 'A', description: 'Toggle AI assistant', context: 'with project' },
   { key: 'G', description: 'Toggle Kanban/Graph view', context: 'with project' },
   { key: ',', description: 'Open settings' },