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.d.ts

@@ -1,8 +1,11 @@
 // Cynhyrchwyd y ffeil hon yn awtomatig. PEIDIWCH Â MODIWL
 // This file is automatically generated. DO NOT EDIT
+import {download} from '../models';
 import {tracklist} from '../models';
 import {context} from '../models';
 
+export function GetDownloadPreferences():Promise<download.AutoDownloadPrefs>;
+
 export function GetFavoritesIconStyle():Promise<string>;
 
 export function GetFavoritesPlaylistID():Promise<number>;
@@ -29,6 +32,8 @@ export function Save():Promise<void>;
 
 export function SetContext(arg1:context.Context):Promise<void>;
 
+export function SetDownloadPreferences(arg1:download.AutoDownloadPrefs):Promise<void>;
+
 export function SetFavoritesIconStyle(arg1:string):Promise<void>;
 
 export function SetFavoritesPlaylistID(arg1:number):Promise<void>;