fix(downloads): file download honours the per-tab account (?account=)

A plain <a> file download can't send the X-Siftlode-Account header, so current_user resolved
it to the session-default account — 404 'Unknown download' when the tab acts as a non-default
wallet account that owns the file. resolved_user_id now also honours a ?account= query param
(the same wallet-gated selection the WebSocket already uses), and downloadFileUrl appends the
active account id. Verified: default account -> 404, ?account=<owner> -> 206 with the right
Content-Disposition.

backend/app/auth.py

@@ -683,7 +683,10 @@ def resolved_user_id(request: Request) -> tuple[int | None, bool]:
     """
     default_id = request.session.get("user_id")
     wallet = request.session.get("account_ids") or []
-    hdr = request.headers.get(ACTIVE_ACCOUNT_HEADER)
+    # Header for normal XHR; ?account= for contexts that can't set headers (WebSocket, and a
+    # plain <a> file download). Both are wallet-gated below, so neither can impersonate an
+    # account that never signed into this browser.
+    hdr = request.headers.get(ACTIVE_ACCOUNT_HEADER) or request.query_params.get("account")
     if hdr:
         try:
             hid = int(hdr)

frontend/src/lib/api.ts

@@ -1044,7 +1044,13 @@ export const api = {
     req(`/api/downloads/${id}/share`, { method: "POST", body: JSON.stringify({ email }) }),
   unshareDownload: (id: number, email: string) =>
     req(`/api/downloads/${id}/share/${encodeURIComponent(email)}`, { method: "DELETE" }),
-  downloadFileUrl: (id: number): string => `/api/downloads/${id}/file`,
+  // A plain <a> navigation can't send the X-Siftlode-Account header, so pass the active
+  // account via ?account= (same wallet-gated selection the WebSocket uses) — otherwise the
+  // download resolves to the session-default account and 404s for a per-tab account's file.
+  downloadFileUrl: (id: number): string => {
+    const a = getActiveAccount();
+    return a != null ? `/api/downloads/${id}/file?account=${a}` : `/api/downloads/${id}/file`;
+  },
 
   // admin
   adminDownloads: (): Promise<DownloadJob[]> => req("/api/admin/downloads"),