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

electron/electron.d.ts

@@ -60,10 +60,6 @@ declare global {
             maximize: () => void
             /** Close the window */
             close: () => void
-            /** Open file dialog and return file path */
-            openFile: () => Promise<string | null>
-            /** Save data to file via save dialog */
-            saveFile: (data: string) => Promise<boolean>
             /** Get proxy configuration */
             getProxy: () => Promise<ProxyConfig>
             /** Set proxy configuration (saves and restarts server) */