fix: display specific validation errors when publish fails with 422

The handlePublish catch block was showing a generic "Failed to publish"
message instead of parsing the 422 response body for validation details.
Now matches the handleSaveDraft error handling pattern.

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

frontend/src/pages/TreeEditorPage.tsx

@@ -349,8 +349,23 @@ export function TreeEditorPage() {
         toast.success('Tree published successfully')
         navigate(`/trees/${newTree.id}/edit`, { replace: true })
       }
-    } catch (err) {
+    } catch (err: unknown) {
       console.error('Failed to publish tree:', err)
+      if (err && typeof err === 'object' && 'response' in err) {
+        const axiosErr = err as { response?: { status?: number; data?: { detail?: string | { message?: string; errors?: Array<string | { message?: string }> } } } }
+        if (axiosErr.response?.status === 422) {
+          const detail = axiosErr.response.data?.detail
+          if (typeof detail === 'object' && detail?.errors) {
+            const messages = detail.errors.map(e => typeof e === 'string' ? e : e.message || 'Unknown error')
+            toast.error(`Cannot publish: ${messages.join(', ')}`)
+          } else if (typeof detail === 'string') {
+            toast.error(`Cannot publish: ${detail}`)
+          } else {
+            toast.error('Tree has validation errors. Fix them before publishing.')
+          }
+          return
+        }
+      }
       toast.error('Failed to publish tree. Please try again.')
     } finally {
       setSaving(false)