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/preload/index.ts

@@ -18,10 +18,6 @@ contextBridge.exposeInMainWorld("electronAPI", {
     maximize: () => ipcRenderer.send("window-maximize"),
     close: () => ipcRenderer.send("window-close"),
 
-    // File operations
-    openFile: () => ipcRenderer.invoke("dialog-open-file"),
-    saveFile: (data: string) => ipcRenderer.invoke("dialog-save-file", data),
-
     // Proxy settings
     getProxy: () => ipcRenderer.invoke("get-proxy"),
     setProxy: (config: { httpProxy?: string; httpsProxy?: string }) =>