[nsaintot]

• New look: a drop zone with an animated globe, glass panels, Instrument
  Sans, a status bar with the server switch, and an empty-state placeholder
  for the Separated list.
• Own title bar on macOS and Windows, with the name and version centred;
  Windows draws its own window buttons and resize grips. Linux unchanged.
• The window opens centred and remembers its position and size.
• Animations run at the same speed at any refresh rate.
• Stemd is capitalised in the app bundle, installer, shortcuts and docs.
• The server switch turns the API off (503) and cancels client jobs; drops
  carry on. A running or queued drop can be stopped from its row.
• A second launch on the same cache folder stops at once, whatever its
  port. Needs Rust 1.89 for the file lock.
• Windows: the MLX GPU thread no longer exits, which froze the window for
  about 3 s while Windows reported the crash.
• Drops: a full queue fails the job instead of leaving it queued forever;
  cached stems survive a failed copy; cache hits skip the queue order.
• The API checks disk space after the cache lookup, so cache hits are
  served on a low disk.
• mDNS registration runs off the UI thread.