refactor(download): rename Want/Request to Request/Download, unify downloads flow, add auto-download guardrails

The durable "I asked for this" record was called Want, and the one-shot
search-and-grab attempt was called Request — names that didn't match
what either actually did. Want is now Request, and the old Request/Item
is now Download/DownloadItem, with a table-rename migration
(download_wants -> download_requests, old download_requests ->
download_downloads) safe against both fresh installs and existing data.

Every anchored manual download now upserts/reuses a durable Request
before running, so a "download now" that finds nothing is picked up by
the background reconciler automatically instead of just failing with
no trace — the gap that caused this session's repeated "no candidates
found" failures on the same album.

Also adds auto-download guardrails (file-size min/max with a preferred
target, allowed file types) that gate what the pipeline may grab
unattended, live-editable from a new settings section. The frontend's
wanted-view becomes downloads-view, with a new Downloads tab showing
attempt/transfer history that previously had no UI at all.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2Agd9af5hE7qzti2ackiS

backend/download/provider_slskd.go

@@ -278,7 +278,7 @@ func (t slskdTransfer) done() (finished, ok bool) {
 // per-folder candidates.  A folder from one peer is the unit a user
 // actually wants: Soulseek has no album concept, but people organise
 // their shares by album directory.
-func (s *slskd) Search(ctx context.Context, req Request) ([]Candidate, error) {
+func (s *slskd) Search(ctx context.Context, dl Download) ([]Candidate, error) {
 	// slskd's search endpoint deserializes id as a .NET Guid server-side,
 	// so it must be a dashed UUID — the app's own newID() (a plain hex
 	// string, used for request/item IDs elsewhere) is rejected with an
@@ -287,7 +287,7 @@ func (s *slskd) Search(ctx context.Context, req Request) ([]Candidate, error) {
 
 	body := map[string]any{
 		"id":         searchID,
-		"searchText": req.SearchText(),
+		"searchText": dl.SearchText(),
 	}
 
 	if err := s.client.post(ctx, "/api/v0/searches", body, nil); err != nil {