fix(clip): clip mode keeps its range, its focus and its keys through the player's own moves

- A mark on the very end seeks a moment short of it; while a clip is marked the end of the item
  neither closes the player nor steps the queue, and Plex's auto-skip waits.
- Plex: Escape and Back peel what is over the picture (info, up-next, the countdown) before the
  clip; Back under a dialog is the dialog's; no range opens while the length is unknown.
- YouTube: the focus goes back to the scissors the user can see, else the player; the panel keeps
  its name, Loop and focus when the view is maximised or restored under it.
- A typed time stays in its field until taken, then shows where it landed; unreadable stays marked
  invalid; a decimal comma is read (ui/TimecodeField, the editor's fields too).
- RangeTrack: a drag is its pointer's, a cancelled drag reports final, an adornment sits beside the
  slider, a caller-chosen focus ring (amber on the clip strip). CropFrame: a drag is its pointer's.
- The editor's remove-cut is a real button at the cut's foot; a segment's keep toggle sits over the
  cut's hit area.

docs/DESIGN.md

@@ -556,16 +556,23 @@ of a full-width panel and nothing on the right.
   around the selection with the two ends (`ui/RangeTrack`: each end a slider — arrows step a tenth,
   Shift a second; drawn 4px, hit 1.5rem), Start / End fields with a tenth back / forward and "to the
   playhead", the length, Play selection (loops), a name, Cancel / Cut clip. On the always-dark chrome the
-  ends are white and the playhead amber (a light theme's accent all but vanished there). The strip's window
+  ends are white, a focused end's ring and the playhead amber (a light theme's accent all but vanished
+  there). A typed time that cannot be read stays in its field, marked invalid; one past the end shows
+  where it landed (decimal point or comma). The strip's window
   follows the selection when it settles, never under a dragging pointer. Plex seeks once a drag is
-  released (a seek may restart the transcode); YouTube follows the drag. Escape closes the panel before the
-  player; the scissors gets the focus back. **Crop (owner's picks K1 + a):** a Crop toggle in the panel puts a
+  released (a seek may restart the transcode); YouTube follows the drag; a mark on the very end seeks a
+  moment short of it, and while a clip is marked the end of the item never closes the player or steps the
+  queue, and Plex's auto-skip waits. Escape closes the panel before the player, but after anything over
+  the picture (Plex: the info, the up-next card, the countdown); ⌫ under a dialog is the dialog's. The
+  scissors the user can see gets the focus back (maximised: the docked bar's), else the player. The panel
+  keeps its name, Loop and focus when the view is maximised or restored under it. **Crop (owner's picks K1 + a):** a Crop toggle in the panel puts a
   frame over the picture (`ui/CropFrame`: on the picture, not its bars; the outside dimmed; drag to move, edges
   and corners to resize, thirds while dragging; the arrows move it, a focused corner's resize it, Shift
   further; focus ring amber) with a shape — Free, 16:9, 9:16, 1:1 (the largest of it the picture holds) — and
   Whole picture; off, the clip is the whole picture. Escape turns the crop off before it closes the panel. A
   press outside the frame is still the video's. A cut is a toast with a Downloads link and the usual
-  "Ready to watch" notification. Limits are the server's: 1 s to 10 min (`lib/clipRange`).
+  "Ready to watch" notification. Limits are the server's: 1 s to 10 min (`lib/clipRange`, held to
+  `downloads/clip.py` by `clipLimits.contract.test.ts`; the "too long" message names the server's cap).
 
 ## Dialogs — _Decided_ (C4, built in S3f)