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

components/chat/TemplatePanel.tsx

@@ -224,7 +224,8 @@ export function TemplatePanel({
             const url = URL.createObjectURL(blob)
             const a = document.createElement("a")
             a.href = url
-            a.download = `templates-${new Date().toISOString().split("T")[0]}.json`
+            // Local date as YYYY-MM-DD (toISOString would give UTC)
+            a.download = `templates-${new Date().toLocaleDateString("sv-SE")}.json`
             document.body.appendChild(a)
             a.click()
             document.body.removeChild(a)