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

frontend/wailsjs/go/config/Config.js

@@ -2,6 +2,10 @@
 // Cynhyrchwyd y ffeil hon yn awtomatig. PEIDIWCH Â MODIWL
 // This file is automatically generated. DO NOT EDIT
 
+export function GetDownloadPreferences() {
+  return window['go']['config']['Config']['GetDownloadPreferences']();
+}
+
 export function GetFavoritesIconStyle() {
   return window['go']['config']['Config']['GetFavoritesIconStyle']();
 }
@@ -54,6 +58,10 @@ export function SetContext(arg1) {
   return window['go']['config']['Config']['SetContext'](arg1);
 }
 
+export function SetDownloadPreferences(arg1) {
+  return window['go']['config']['Config']['SetDownloadPreferences'](arg1);
+}
+
 export function SetFavoritesIconStyle(arg1) {
   return window['go']['config']['Config']['SetFavoritesIconStyle'](arg1);
 }