fix: suppress Improv WiFi step so only Close button shows after flash

Add new_install_improv_wait_ms: 0 to manifest — prevents esp-web-tools
from waiting for Improv WiFi provisioning response after installation,
so the completion dialog shows only Close instead of Next.

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

backend/server.js

@@ -155,6 +155,7 @@ app.get('/api/firmware/:id/manifest', (req, res) => {
     name:    firmware.name,
     version: firmware.version,
     new_install_prompt_erase:   true,
+    new_install_improv_wait_ms: 0,    // Improv WiFi 단계 건너뜀 → 완료 후 Close 버튼만 표시
     builds: [{
       chipFamily: firmware.chipFamily,
       parts: firmware.parts.map(p => ({