feat: remove every subscription from Settings, and reopen the last video

Remove all sits beside Import in Settings, since emptying the list is
the other half of replacing it. It asks first and counts what goes:
210 channels, 3339 videos, 50 downloaded files, in those words. Nothing
changes on YouTube, and the confirmation says so — importing or reading
the list again brings it all back.

It takes the same path an import does when nothing survives, so a video
saved on its own from the menu bar is left alone: its channel was never
a subscription, and this is about subscriptions.

The app also reopens whatever was playing when it last closed, if that
video is still in the feed and still playable — offline, one that was
streaming is not. The note is made while the player is open and dropped
when it is closed, so a restart only reopens something you were in the
middle of, never something you had deliberately finished with. Verified
both ways: quitting mid-video came back to it, closing the player first
came back to the feed.

The panel's Remove all downloads is gone; it was a misreading of the
request.

src/api.ts

@@ -73,6 +73,12 @@ export const previewScrapedImport = (channels: ScrapeResult["channels"]) =>
 export const importScraped = (channels: ScrapeResult["channels"]) =>
   invoke<number>("import_scraped", { channels });
 
+/** What emptying the subscription list would take with it. */
+export const previewRemoveAllSubscriptions = () =>
+  invoke<ImportPreview>("preview_remove_all_subscriptions");
+
+export const removeAllSubscriptions = () => invoke<number>("remove_all_subscriptions");
+
 /** The menu bar panel's own actions. */
 export const traySaveVideo = () => invoke<void>("tray_save_video");
 export const showMainWindow = () => invoke<void>("show_main_window");