[WillTDA]

Three bugs found on beta.4. All three are in the app; electron-kit has no part in them, so the kit stays at 0.1.0-beta.3.

• Cancel flickered under the pointer. renderCardActions cleared and rebuilt the card's buttons on every progress tick, so the hovered Cancel was replaced many times a second. The buttons are now only rebuilt when the status or output changes.
• The Last frame box stayed empty. A container can run past its video's last frame when the audio is longer (the reported file's container ends at 3:10.334, its picture at 3:10.284). ffmpeg decodes nothing in that gap and exits cleanly, so no image came back. The preview handler now steps back 0.25s, 1s, then 5s until it finds a picture. A real decode failure still gives no preview.
• Advanced Options opened and "applied" on a card being converted. Settings and format are sent when Convert is pressed, so the edits never took effect, but the toast said they had. Cards in the batch, queued as well as running, now lock their tune button, image-count line and format select until their result comes back. Apply also skips them and only counts the files it changed.

Version 2.0.0-beta.5. npm test: 198 passing. A new section in docs/MANUAL-TESTING.md covers all three; it hasn't been run yet.