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/timecode.test.ts

@@ -0,0 +1,36 @@
+import { describe, expect, it } from "vitest";
+import { formatTimecode, parseTimecode } from "./timecode";
+
+describe("formatTimecode", () => {
+  it("is m:ss.d below an hour and h:mm:ss.d from one", () => {
+    expect(formatTimecode(12.44)).toBe("0:12.4");
+    expect(formatTimecode(75)).toBe("1:15.0");
+    expect(formatTimecode(3723.4)).toBe("1:02:03.4");
+  });
+
+  it("shows hours when the item reaches one, so an item's fields agree", () => {
+    expect(formatTimecode(65, true)).toBe("0:01:05.0");
+  });
+
+  it("carries a rounded tenth into the next second, never a 60.0", () => {
+    expect(formatTimecode(59.96)).toBe("1:00.0");
+  });
+
+  it("reads a bad clock as zero", () => {
+    expect(formatTimecode(NaN)).toBe("0:00.0");
+    expect(formatTimecode(-3)).toBe("0:00.0");
+  });
+});
+
+describe("parseTimecode", () => {
+  it("reads every shape it writes, and bare seconds", () => {
+    expect(parseTimecode("0:12.4")).toBeCloseTo(12.4);
+    expect(parseTimecode("1:02:03.4")).toBeCloseTo(3723.4);
+    expect(parseTimecode(" 90.5 ")).toBeCloseTo(90.5);
+  });
+
+  it("is null for what is not a timecode", () => {
+    for (const v of ["", "abc", "1::2", "1:2:3:4", "-5", "1:-2"])
+      expect(parseTimecode(v)).toBeNull();
+  });
+});