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/types.ts

@@ -75,3 +75,10 @@ export interface DownloadStateEvent {
   error: string | null;
   path: string | null;
 }
+
+export interface ImportPreview {
+  incoming: number;
+  removed_channels: number;
+  removed_videos: number;
+  removed_downloads: number;
+}