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/AssistantPanel.tsx

@@ -11,6 +11,7 @@ import { X, Bot } from 'lucide-react'
 import { AssistantChat } from './AssistantChat'
 import { useConversation } from '../hooks/useConversations'
 import type { ChatMessage } from '../lib/types'
+import { Button } from '@/components/ui/button'
 
 interface AssistantPanelProps {
   projectName: string
@@ -103,45 +104,37 @@ export function AssistantPanel({ projectName, isOpen, onClose }: AssistantPanelP
         className={`
           fixed right-0 top-0 bottom-0 z-50
           w-[400px] max-w-[90vw]
-          bg-neo-card
-          border-l-4 border-[var(--color-neo-border)]
+          bg-card
+          border-l border-border
           transform transition-transform duration-300 ease-out
-          flex flex-col
+          flex flex-col shadow-xl
           ${isOpen ? 'translate-x-0' : 'translate-x-full'}
         `}
-        style={{ boxShadow: 'var(--shadow-neo-left-lg)' }}
         role="dialog"
         aria-label="Project Assistant"
         aria-hidden={!isOpen}
       >
         {/* Header */}
-        <div className="flex items-center justify-between px-4 py-3 border-b-3 border-neo-border bg-neo-progress">
+        <div className="flex items-center justify-between px-4 py-3 border-b border-border bg-primary text-primary-foreground">
           <div className="flex items-center gap-2">
-            <div
-              className="bg-neo-card border-2 border-neo-border p-1.5"
-              style={{ boxShadow: 'var(--shadow-neo-sm)' }}
-            >
+            <div className="bg-card text-foreground border border-border p-1.5 rounded">
               <Bot size={18} />
             </div>
             <div>
-              <h2 className="font-display font-bold text-neo-text-on-bright">Project Assistant</h2>
-              <p className="text-xs text-neo-text-on-bright opacity-80 font-mono">{projectName}</p>
+              <h2 className="font-semibold">Project Assistant</h2>
+              <p className="text-xs opacity-80 font-mono">{projectName}</p>
             </div>
           </div>
-          <button
+          <Button
+            variant="ghost"
+            size="icon"
             onClick={onClose}
-            className="
-              neo-btn neo-btn-ghost
-              p-2
-              bg-[var(--color-neo-card)] border-[var(--color-neo-border)]
-              hover:bg-[var(--color-neo-bg)]
-              text-[var(--color-neo-text)]
-            "
+            className="text-primary-foreground hover:bg-primary-foreground/20"
             title="Close Assistant (Press A)"
             aria-label="Close Assistant"
           >
             <X size={18} />
-          </button>
+          </Button>
         </div>
 
         {/* Chat area */}