feat: Takeout walkthrough, design-system restyle, replacing import

Adds a seven-step in-app guide to exporting subscriptions from Google
Takeout, with the real URLs opened in the system browser.

Restyles the app onto the supplied design system: slate/sky palette,
9-15px type ladder, outline-first controls, tiered radii, borders for
separation and shadows only for elevation. Light and dark are both
designed, with a System/Light/Dark control and a pre-paint script so
the window does not flash light on a dark machine.

Importing now replaces the subscription list rather than merging, per
request. Because that can delete downloaded files, a confirmation
dialog names exactly what will go first.

src/api.ts

@@ -7,6 +7,7 @@ import type {
   DownloadStateEvent,
   FeedFilter,
   FeedItem,
+  ImportPreview,
   Prereqs,
   RefreshProgress,
   RefreshSummary,
@@ -39,16 +40,22 @@ export const openExternal = (url: string) =>
   invoke<void>("open_external", { url });
 
 /** Opens the native file picker for a Takeout subscriptions.csv. */
-export async function pickAndImportTakeout(): Promise<number | null> {
+export async function pickTakeoutFile(): Promise<string | null> {
   const path = await open({
     multiple: false,
     directory: false,
     filters: [{ name: "Takeout subscriptions", extensions: ["csv"] }],
   });
-  if (typeof path !== "string") return null;
-  return invoke<number>("import_takeout_csv", { path });
+  return typeof path === "string" ? path : null;
 }
 
+export const previewTakeoutImport = (path: string) =>
+  invoke<ImportPreview>("preview_takeout_import", { path });
+
+/** Replaces the whole subscription list. Confirm with the user first. */
+export const importTakeoutCsv = (path: string) =>
+  invoke<number>("import_takeout_csv", { path });
+
 export async function pickLibraryFolder(): Promise<string | null> {
   const path = await open({ directory: true, multiple: false });
   if (typeof path !== "string") return null;