feat: pick a folder per download with D (#54)

The o key sets one global download folder, so routing a single result
somewhere else (a movie to the NAS, a game to the big drive) meant
changing the setting and changing it back. D on a result now opens a
"download to" prompt for just that item; d stays a one-keypress
download to the default.

The queue already stores a dir per item, so resume, re-seed and
download-again honor the choice with no new plumbing. The prompt
reuses FolderPrompt (new title prop), pre-fills with the last D folder
this session so batches cost one typed path, and nothing new is
persisted. The ? sheet gains d/D entries (d was missing entirely) and
the README now mentions both o and D, which it never documented.

One behavior fix rides along because D exposed it: queue.add() used to
ignore the requested dir when re-adding a failed item, so redirecting
a failed download (the disk-full case D exists for) silently retried
into the old folder. add() now adopts the requested dir on failed
retry, dropping resume progress only when the folder actually changed;
covered in queue.add.test.ts with the engine mocked. The f retry key
goes through queue.retry() and keeps the item's own folder, unchanged.
D on an already-active item says "Already in queue" instead of
claiming a folder that wouldn't be used.

scripts/render-previews-impl.tsx

@@ -97,6 +97,7 @@ function makeStore(
     seedFocus: null,
     setSeedFocus: noop,
     startDownload: noop,
+    requestDownloadTo: noop,
     copyMagnet: noop,
     openDownloadFolder: noop,
     notice: null,