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/lib/api/downloads.ts

@@ -51,6 +51,15 @@ export type DownloadJobKind = "download" | "edit" | "upload" | "clip";
 
 // Phase-2 editor recipe. A single `trim` (one output file) or a `segments` cut-list joined into
 // one file. `accurate` re-encodes for a frame-accurate cut (a crop always re-encodes).
+export interface ClipRequest {
+  source: "youtube" | "plex";
+  ref: string;
+  start_s: number;
+  end_s: number;
+  audio_track?: number;
+  display_name?: string;
+}
+
 export interface EditSpec {
   trim?: { start_s: number; end_s?: number };
   segments?: { start_s: number; end_s: number }[];
@@ -226,6 +235,10 @@ export const downloadsApi = {
     display_name?: string;
   }): Promise<DownloadJob> =>
     req("/api/downloads/edit", { method: "POST", body: JSON.stringify(body) }),
+  // An ad-hoc clip marked in a player: a YouTube video's or a Plex item's range, cut into a new
+  // download of its own (`audio_track`: the Plex audio track that was playing).
+  enqueueClip: (body: ClipRequest): Promise<DownloadJob> =>
+    req("/api/downloads/clip", { method: "POST", body: JSON.stringify(body) }),
   downloadStoryboard: (id: number): Promise<StoryboardMeta> =>
     req(`/api/downloads/${id}/storyboard`),
   downloads: (): Promise<DownloadJob[]> => req("/api/downloads"),