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/keymap.ts

@@ -166,6 +166,11 @@ export const SHORTCUTS = [
   { id: "nextVideo", scope: "ytPlayer", keys: ["Shift+ArrowRight"], labelKey: "keys.video" },
   { id: "volumeUp", scope: "ytPlayer", keys: ["ArrowUp"], labelKey: "keys.volume" },
   { id: "volumeDown", scope: "ytPlayer", keys: ["ArrowDown"], labelKey: "keys.volume" },
+  // Clip mode (the clip panel): `c` opens / closes it, `[` and `]` mark its ends at the playhead
+  // (opening it when it is shut).
+  { id: "clip", scope: "ytPlayer", keys: ["c"], labelKey: "keys.clip" },
+  { id: "markIn", scope: "ytPlayer", keys: ["["], labelKey: "keys.clipMark" },
+  { id: "markOut", scope: "ytPlayer", keys: ["]"], labelKey: "keys.clipMark" },
   {
     id: "close",
     scope: "ytPlayer",
@@ -202,6 +207,9 @@ export const SHORTCUTS = [
   { id: "subtitle", scope: "plexPlayer", keys: ["s"], labelKey: "keys.subtitle" },
   { id: "info", scope: "plexPlayer", keys: ["i"], labelKey: "keys.info" },
   { id: "stats", scope: "plexPlayer", keys: ["d"], labelKey: "keys.stats" },
+  { id: "clip", scope: "plexPlayer", keys: ["c"], labelKey: "keys.clip" },
+  { id: "markIn", scope: "plexPlayer", keys: ["["], labelKey: "keys.clipMark" },
+  { id: "markOut", scope: "plexPlayer", keys: ["]"], labelKey: "keys.clipMark" },
   // Two rows, one help line: ⌫ is the plain "stop & out", Escape is the layer-aware cascade (defer to
   // anything above → cancel auto-skip → close the topmost player overlay → close). Sharing a labelKey
   // keeps them a single "⌫ Esc  Stop & go back" row in the sheet.
@@ -266,6 +274,11 @@ export function isActivationTarget(e: KeyboardEvent): boolean {
   return tag === "button" || tag === "a";
 }
 
+/** One printable character that is neither a letter nor a digit (`[`, `]`, `,`). */
+function isSymbol(name: string): boolean {
+  return [...name].length === 1 && !/[\p{L}\p{N}\s]/u.test(name);
+}
+
 function keyMatches(e: KeyboardEvent, name: string): boolean {
   if (name === "Space") return e.key === " " || e.code === "Space";
   if (name.length === 1) return e.key.toLowerCase() === name.toLowerCase();
@@ -280,6 +293,10 @@ export function matches(e: KeyboardEvent, chord: string): boolean {
   const parts = chord.split("+");
   const key = parts[parts.length - 1] ?? "";
   const mods = parts.slice(0, -1).map((m) => m.toLowerCase());
+  // A symbol the layout types through AltGr (the HU `[` is AltGr+F, Ctrl+Alt on Windows) is that
+  // symbol: a bare-symbol row answers it. A letter or a Mod row never does — AltGr+K is not Ctrl+K.
+  if (mods.length === 0 && isSymbol(key) && e.getModifierState("AltGraph"))
+    return keyMatches(e, key);
   if ((e.ctrlKey || e.metaKey) !== mods.includes("mod")) return false;
   if (e.shiftKey !== mods.includes("shift")) return false;
   if (e.altKey !== mods.includes("alt")) return false;