Add dark mode, export preview, and keyboard navigation

- Add theme store with light/dark/system modes and ThemeToggle component
- Prevent flash of wrong theme on initial load via inline script
- Add ExportPreviewModal for previewing session exports before download
- Add copy-to-clipboard functionality to session export
- Implement keyboard shortcuts for tree navigation (1-9 options, Esc back, Enter continue)
- Display keyboard hints in tree navigation UI
- Fix findNode to safely handle undefined structure parameter
- Update page title to "Apoklisis"

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

frontend/src/components/session/ExportPreviewModal.tsx

@@ -0,0 +1,103 @@
+import { useState } from 'react'
+import { Copy, Download, Check } from 'lucide-react'
+import { Modal } from '@/components/common/Modal'
+import { cn } from '@/lib/utils'
+
+interface ExportPreviewModalProps {
+  isOpen: boolean
+  onClose: () => void
+  content: string
+  filename: string
+  format: 'markdown' | 'text' | 'html'
+  onDownload: () => void
+}
+
+export function ExportPreviewModal({
+  isOpen,
+  onClose,
+  content,
+  filename,
+  format,
+  onDownload,
+}: ExportPreviewModalProps) {
+  const [copied, setCopied] = useState(false)
+
+  const handleCopy = async () => {
+    try {
+      await navigator.clipboard.writeText(content)
+      setCopied(true)
+      setTimeout(() => setCopied(false), 2000)
+    } catch (err) {
+      console.error('Failed to copy:', err)
+    }
+  }
+
+  const handleDownload = () => {
+    onDownload()
+    onClose()
+  }
+
+  // Reset copied state when modal closes
+  const handleClose = () => {
+    setCopied(false)
+    onClose()
+  }
+
+  return (
+    <Modal isOpen={isOpen} onClose={handleClose} title="Export Preview" size="xl">
+      {/* Filename and format info */}
+      <p className="mb-3 text-sm text-muted-foreground">
+        Filename: <span className="font-mono text-foreground">{filename}</span>
+        <span className="ml-3 rounded bg-secondary px-2 py-0.5 text-xs">
+          {format === 'markdown' ? 'Markdown' : format === 'html' ? 'HTML' : 'Plain Text'}
+        </span>
+      </p>
+
+      {/* Content Preview */}
+      <div
+        className={cn(
+          'max-h-96 overflow-auto rounded-md border border-input bg-muted/50 p-4',
+          'font-mono text-sm text-foreground'
+        )}
+      >
+        <pre className="whitespace-pre-wrap">{content}</pre>
+      </div>
+
+      {/* Actions */}
+      <div className="mt-4 flex items-center justify-end gap-2">
+        <button
+          onClick={handleCopy}
+          className={cn(
+            'flex items-center gap-2 rounded-md border border-input px-3 py-2 text-sm font-medium',
+            'bg-background text-foreground hover:bg-accent',
+            'focus:outline-none focus:ring-2 focus:ring-ring'
+          )}
+        >
+          {copied ? (
+            <>
+              <Check className="h-4 w-4 text-green-500" />
+              Copied!
+            </>
+          ) : (
+            <>
+              <Copy className="h-4 w-4" />
+              Copy to Clipboard
+            </>
+          )}
+        </button>
+        <button
+          onClick={handleDownload}
+          className={cn(
+            'flex items-center gap-2 rounded-md bg-primary px-3 py-2 text-sm font-medium text-primary-foreground',
+            'hover:bg-primary/90 focus:outline-none focus:ring-2 focus:ring-ring'
+          )}
+        >
+          <Download className="h-4 w-4" />
+          Download
+        </button>
+      </div>
+    </Modal>
+  )
+}
+
+export default ExportPreviewModal