Let the reader have the captured pictures taken again

A frame taken out of a video is kept for good: that is what makes a dead
favorite cheap to scroll past, and it is also the one thing that can go wrong
with no way out. A picture from the wrong moment, or one kept by a version of
this that would keep a frame with nothing in it, stays on the card for the life
of the item.

So Settings grows a Captured Thumbnails section: what is kept and how much room
it takes, and a button that throws the lot away. The cards on screen go back to
the start of the thumbnail ladder rather than waiting to be scrolled out of the
window and back, which is where a fresh capture comes from, and the session's
memory of which items there was no point asking about goes with the pictures --
otherwise "take them again" would quietly skip every item that failed once.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MPZFnNdHbPGDTqQUNiE4ZN

frontend/js/posters.js

@@ -200,10 +200,18 @@ App.posters = App.posters || {};
         writeIndex();
     };
 
+    // Throws every captured picture away, and with it this session's memory of
+    // which items there was no point asking about. That memory is the whole
+    // reason a dead favorite is cheap to scroll past, so it only goes when the
+    // reader asks for the pictures to be taken again (Settings -> Captured
+    // Thumbnails) -- otherwise an item that failed once would be retried on
+    // every mount for the rest of the session.
     App.posters.clear = function() {
         readIndex().forEach(drop);
         index = [];
         writeIndex();
+        failed.clear();
+        rechecks.clear();
     };
 
     App.posters.stats = function() {