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-input.tsx

@@ -642,9 +642,8 @@ export const ChatInput = forwardRef<ChatInputRef, ChatInputProps>(
                             dict.save.savedSuccessfully,
                         )
                     }
-                    defaultFilename={`diagram-${new Date()
-                        .toISOString()
-                        .slice(0, 10)}`}
+                    // Local date as YYYY-MM-DD (toISOString would give UTC)
+                    defaultFilename={`diagram-${new Date().toLocaleDateString("sv-SE")}`}
                 />
                 {onUrlChange && (
                     <UrlInputDialog