fix(memory-echo): fix fusion route config and single close button on comparison modal

- Fix fusion route using same broken prisma.systemConfig.findFirst()
  instead of getSystemConfig() — this caused all fusion requests to fail
- Replace duplicate close buttons (Radix auto + custom) with single
  styled close button in comparison modal header

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

keep-notes/app/api/ai/echo/fusion/route.ts

@@ -1,6 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server'
 import { auth } from '@/auth'
-import { getAIProvider } from '@/lib/ai/factory'
+import { getChatProvider } from '@/lib/ai/factory'
+import { getSystemConfig } from '@/lib/config'
 import prisma from '@/lib/prisma'
 
 /**
@@ -50,8 +51,8 @@ export async function POST(req: NextRequest) {
     }
 
     // Get AI provider
-    const config = await prisma.systemConfig.findFirst()
-    const provider = getAIProvider(config || undefined)
+    const config = await getSystemConfig()
+    const provider = getChatProvider(config)
 
     // Build fusion prompt
     const notesDescriptions = notes.map((note, index) => {