refactor(ui): migrate to shadcn/ui components and fix scroll issues

Migrate UI component library from custom implementations to shadcn/ui:
- Add shadcn/ui primitives (Button, Card, Dialog, Input, etc.)
- Replace custom styles with Tailwind CSS v4 theme configuration
- Remove custom-theme.css in favor of globals.css with @theme directive

Fix scroll overflow issues in multiple components:
- ProjectSelector: "New Project" button no longer overlays project list
- FolderBrowser: folder list now scrolls properly within modal
- AgentCard: log modal content stays within bounds
- ConversationHistory: conversation list scrolls correctly
- KanbanColumn: feature cards scroll within fixed height
- ScheduleModal: schedule form content scrolls properly

Key technical changes:
- Replace ScrollArea component with native overflow-y-auto divs
- Add min-h-0 to flex containers to allow proper shrinking
- Restructure dropdown layouts with flex-col for fixed footers

New files:
- ui/components.json (shadcn/ui configuration)
- ui/src/components/ui/* (20 UI primitive components)
- ui/src/lib/utils.ts (cn utility for class merging)
- ui/tsconfig.app.json (app-specific TypeScript config)

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

ui/src/components/ViewToggle.tsx

@@ -1,4 +1,5 @@
 import { LayoutGrid, GitBranch } from 'lucide-react'
+import { Button } from '@/components/ui/button'
 
 export type ViewMode = 'kanban' | 'graph'
 
@@ -12,35 +13,25 @@ interface ViewToggleProps {
  */
 export function ViewToggle({ viewMode, onViewModeChange }: ViewToggleProps) {
   return (
-    <div className="inline-flex rounded-lg border-2 border-neo-border p-1 bg-white shadow-neo-sm">
-      <button
+    <div className="inline-flex rounded-lg border p-1 bg-background">
+      <Button
+        variant={viewMode === 'kanban' ? 'default' : 'ghost'}
+        size="sm"
         onClick={() => onViewModeChange('kanban')}
-        className={`
-          flex items-center gap-1.5 px-3 py-1.5 rounded-md font-medium text-sm transition-all
-          ${viewMode === 'kanban'
-            ? 'bg-neo-accent text-white shadow-neo-sm'
-            : 'text-neo-text hover:bg-neo-neutral-100'
-          }
-        `}
         title="Kanban View"
       >
         <LayoutGrid size={16} />
-        <span>Kanban</span>
-      </button>
-      <button
+        Kanban
+      </Button>
+      <Button
+        variant={viewMode === 'graph' ? 'default' : 'ghost'}
+        size="sm"
         onClick={() => onViewModeChange('graph')}
-        className={`
-          flex items-center gap-1.5 px-3 py-1.5 rounded-md font-medium text-sm transition-all
-          ${viewMode === 'graph'
-            ? 'bg-neo-accent text-white shadow-neo-sm'
-            : 'text-neo-text hover:bg-neo-neutral-100'
-          }
-        `}
         title="Dependency Graph View"
       >
         <GitBranch size={16} />
-        <span>Graph</span>
-      </button>
+        Graph
+      </Button>
     </div>
   )
 }