fix(next): add missing 'use client' to TestConnectionButton and KeyValueField

Both components use useState/useEffect/useCallback/useRef but were
missing the 'use client' directive. Without it Next.js App Router
renders them as server HTML — React never hydrates them and event
handlers are silently dropped.

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

canvas/src/components/ui/KeyValueField.tsx

@@ -1,3 +1,5 @@
+'use client';
+
 import { useState, useCallback, useRef, useEffect, type ChangeEvent } from 'react';
 import { RevealToggle } from './RevealToggle';
 

canvas/src/components/ui/TestConnectionButton.tsx

@@ -1,3 +1,5 @@
+'use client';
+
 import { useState, useCallback, useRef, useEffect } from 'react';
 import type { TestConnectionState, SecretGroup } from '@/types/secrets';
 import { validateSecret } from '@/lib/api/secrets';