fix: log Editable SVG saves, use local dates in file names, drop unused Electron dialogs

- /api/log-save accepted only drawio, png and svg, so saving as Editable
  SVG (xmlsvg) got a 400 and was never recorded in Langfuse
- The default download name and the template export name used the UTC
  date, which is the previous day on mornings in East Asia
- Remove the Electron openFile/saveFile IPC handlers, their preload
  bindings and types; nothing in the app calls them

app/api/log-save/route.ts

@@ -4,7 +4,7 @@ import { getLangfuseClient } from "@/lib/langfuse"
 
 const saveSchema = z.object({
     filename: z.string().min(1).max(255),
-    format: z.enum(["drawio", "png", "svg"]),
+    format: z.enum(["drawio", "png", "svg", "xmlsvg"]),
     sessionId: z.string().min(1).max(200).optional(),
 })