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/index.css

@@ -142,6 +142,18 @@
   border-color: var(--edge-strong);
 }
 
+/* A form on the always-dark player chrome (the clip panel's fields): the neutral tokens the field
+   primitives read are re-pointed to white-on-chrome for this subtree, so a light theme's dark text
+   never lands on the dark bar. The accent stays the theme's. */
+@utility media-scope {
+  --fg: #ffffff;
+  --muted: rgb(255 255 255 / 0.62);
+  --bg: var(--media-chrome);
+  --border: rgb(255 255 255 / 0.16);
+  --edge-strong: rgb(255 255 255 / 0.4);
+  color: var(--fg);
+}
+
 /* The reading measure for prose (a synopsis, a biography, a channel's description): ~65 characters
    a line, inside the 45–75 a reader follows back to the next line's start. A full-width page ran
    one to 110 (owner: "shorter lines are easier to follow"). Only where the container can
@@ -280,6 +292,7 @@
   a:focus-visible,
   [role="button"]:focus-visible,
   [role="tab"]:focus-visible,
+  [role="slider"]:focus-visible,
   summary:focus-visible {
     /* Solid accent: the earlier 60% mix all but vanished on the dark glass. */
     outline: 2px solid var(--accent);
@@ -1191,9 +1204,10 @@ html[data-theme="light"] {
 }
 /* …and it only fills the viewport while no ancestor has a filter: a `backdrop-filter` makes that
    ancestor the fixed stage's containing block. The modal's card is a blurred `.glass` panel,
-   so while the stage is maximised the card drops its blur — the stage covers the card anyway. `html `
+   so while the stage is maximised the card drops its blur — the stage covers the card anyway. Any
+   depth: the stage sits in a wrapper with the clip panel. `html `
    outranks `html[data-theme="dark"] .glass`, which sets the blur again (lib/playerStage.contract.test.ts). */
-html .glass:has(> .player-stage--max) {
+html .glass:has(.player-stage--max) {
   backdrop-filter: none;
   -webkit-backdrop-filter: none;
 }