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

tests/unit/log-save-route.test.ts

@@ -0,0 +1,42 @@
+// @vitest-environment node
+import { describe, expect, it, vi } from "vitest"
+import { POST as logSave } from "@/app/api/log-save/route"
+
+// A Langfuse client with a trace to attach the save score to
+const batch = vi.fn(async () => ({}))
+vi.mock("@/lib/langfuse", () => ({
+    getLangfuseClient: () => ({
+        api: {
+            trace: { list: async () => ({ data: [{ id: "trace-1" }] }) },
+            ingestion: { batch },
+        },
+    }),
+}))
+
+function post(body: unknown): Request {
+    return new Request("http://localhost/api/log-save", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify(body),
+    })
+}
+
+describe("POST /api/log-save", () => {
+    it.each(["drawio", "png", "svg", "xmlsvg"])(
+        "logs a save in %s format",
+        async (format) => {
+            const res = await logSave(
+                post({ filename: "diagram", format, sessionId: "s1" }),
+            )
+            expect(res.status).toBe(200)
+            expect(await res.json()).toEqual({ success: true, logged: true })
+        },
+    )
+
+    it("rejects an unknown format", async () => {
+        const res = await logSave(
+            post({ filename: "diagram", format: "pdf", sessionId: "s1" }),
+        )
+        expect(res.status).toBe(400)
+    })
+})