feat(player): clip mode - mark a range of what is playing and cut it into a download

Both players get a scissors in their control row (Plex: the control row; YouTube: the actions and
the docked bar), `c`, and `[` / `]` to mark at the playhead. The clip panel lives in the player, so
the watched video is the preview: an overview of the whole item and a zoomed strip around the
selection whose ends are sliders (arrows step a tenth, Shift a second), Start / End fields with
nudges and "to the playhead", the length, a looping Play selection, a name, Cut. Escape closes the
panel before the player and the focus goes back to the scissors. A cut is a toast with a Downloads
link plus the usual Ready notification.

- ui/RangeTrack + RulerTicks: the shared, domain-free time track (regions, keyboard-steppable
  handles with a 1.5rem hit area, a playhead, a background slot, over a window of the item). The
  Downloads editor's cuts moved onto it, so they are keyboard-steppable now.
- lib/clipRange holds the marking rules (1 s to 10 min, the server's), lib/timecode the editing
  timecode the editor's private tc/parseTc became.
- keymap: a symbol typed through AltGr answers its bare-symbol row - on the HU layout `[` is
  AltGr+F, which arrives as Ctrl+Alt; letter and Mod rows still never match Ctrl+Alt.
- The YouTube stage sits in a wrapper with the panel, so the maximised stage's blur escape matches
  at any depth.
- media-scope: fields on the always-dark player chrome read white in a light theme too.

frontend/src/components/plexPlayerControls.tsx

@@ -1,4 +1,4 @@
-import { type ReactNode } from "react";
+import { type ReactNode, type Ref } from "react";
 import { useTranslation } from "react-i18next";
 import { RotateCcw } from "lucide-react";
 import { IconButton } from "./ui/IconButton";
@@ -96,11 +96,13 @@ export function Ctrl({
   onClick,
   disabled,
   align = "center",
+  buttonRef,
   children,
 }: {
   label: string;
   onClick: () => void;
   disabled?: boolean;
+  buttonRef?: Ref<HTMLButtonElement>;
   // Tooltip horizontal anchor: "start"/"end" for edge buttons so it doesn't clip off the viewport.
   align?: "start" | "center" | "end";
   children: ReactNode;
@@ -114,6 +116,7 @@ export function Ctrl({
   return (
     <div className="relative flex group/ctrl">
       <IconButton
+        ref={buttonRef}
         variant="overlay"
         size="player"
         round