feat(design): the field family, the dropdown trigger, native controls, the no-raw-input lint rule (E28 S3c)

- ui/Input, Textarea, InnerInput + fieldClass (A4): the >= 3:1 edge, a 2px accent
  focus edge, the danger edge for invalid, md/sm/xs/heading rungs, 1rem and 44 on touch
- ui/SelectTrigger (A5): SortControl, FilterSelect, the view switcher and the date
  filter on one trigger, a pill in a toolbar, r8 in a form
- ui/NativeControls (A6): Checkbox, CheckRow, Slider, ColorInput
- every raw input swept (~100 sites); inputCls retired; local/input-primitive
- useReturnFocus: an in-place editor hands the focus back to its trigger on
  Enter/Escape (the playlist and Plex title renames, the priority number);
  the Plex playlist rename gains Escape and an accessible name
- back to top: a round 48 FAB (owner decision); DESIGN.md A2 and the log

frontend/src/components/DownloadCenter.tsx

@@ -37,13 +37,13 @@ import { api, type DownloadJob } from "../lib/api";
 import { notifyActionError } from "../lib/actionErrors";
 import { invalidateDownloads, quickShareLink } from "../lib/downloads";
 import { libraryRowActions, sharedRowActions } from "../lib/downloadActions";
-import { inputCls } from "./ui/form";
 import { Button } from "./ui/Button";
 import { IconButton } from "./ui/IconButton";
 import { DownloadRow, IconBtn, UsageBar } from "./downloadRow";
 import { MetaEditModal, ErrorModal } from "./downloadModals";
 import { AdminSystem } from "./downloadAdmin";
 import NavGrid from "./ui/NavGrid";
+import { Input } from "./ui/Input";
 
 export default function DownloadCenter() {
   const me = useMe();
@@ -189,13 +189,13 @@ export default function DownloadCenter() {
         {tab === "queue" && (
           <>
             <div className="flex gap-2 mb-4">
-              <input
+              <Input
+                className="w-full"
                 value={addUrl}
                 onChange={(e) => setAddUrl(e.target.value)}
                 onFocus={() => pasteFromClipboard()}
                 onKeyDown={(e) => e.key === "Enter" && addUrl.trim() && setAddSource(addUrl.trim())}
                 placeholder={t("downloads.page.addUrl")}
-                className={inputCls}
               />
               <IconButton
                 label={t("downloads.page.paste")}